I'm currently enjoying the second year of a serious relationship with an ISTP, and provided no unforeseen disaster separates us, this is probably the end of my romantic road. I feel like I have a fair amount to share with regard to my situation and why - for me, anyway - this combination works like a charm!
When It's Working
We get each other. Our cognitive functions may be rearranged, but they are the same cognitive functions nonetheless; I cover his weaknesses and he covers mine. It is as though we see more or less the same image, but through slightly different lenses or filters, so that we can contribute to each other's worldviews harmoniously, sharing and comparing almost always without disagreement. This process is not boring to me, and I don't feel like I'm dating myself at all. We are different people, but our preferences of communication style and methods for processing our environment are similar enough that we can understand each other easily.
I met my ISTP nine years ago at a summer orientation session for our chosen major at our chosen university, but due to the volume of introversion within each of us, we never grew especially close until much later. We did, however, help each other out during those scary school years and remain on mutual radar long after we graduated. He was mysterious and interesting, the quiet boy who was so nice to me during orientation but whom I could never quite figure out - and I was attracted to that. Whether intentionally or not, people are typically very open and easy to read insofar as I'm concerned, but this guy was both a puzzle and something familiar. The intrigue did not go unnoticed for him, either. After several nights of (consciously? unconsciously?) seeking out each other's company on Facebook chat at three in the morning, I mentioned that I would be taking a road trip and passing by his town should he desire to meet for a drink. This was the catalyst that finally set everything off, once and for all, though we were hesitant to call ourselves an official pair for a long time.
Mutual introversion, then, can work for or against you. I happen to prefer how slowly things transpired in my own relationship. The glacial pace enabled me to be more sure of my feelings as opposed to jumping into (and out of) a new, exciting sexual venture for the thrill of it, as my younger self was especially prone to. More than that, our relationship has never felt anything other than genuine; we were friends first, at our core, and we happened to be lucky enough to feel a mutual chemistry on top of that. I trust him with my life, and I do not doubt that he feels the same of me.
There are also the practical applications: we understand the need for time alone. Space is a good thing. Being alone is awesome. We respect our tandem need for solo hours and there are no hard feelings; neither of us is clingy and the claustrophobia that previous relationships imparted in me is not present now. I can't emphasize enough how wonderful this is. I get to be myself, by myself, and then I feel perfectly relaxed and balanced, comfortable in my own skin when we cuddle up at night.
Until I met my ISTP, I've never quite encountered anybody else (in person, anyway) who shared my need to keep myself separate from the world while also constantly indulging in it, immersed in it, playing in it. It's a contradiction, one that I struggle to explain sometimes because the feeling is so large. Essentially, I believe that - as @small.wonder already pointed out - there is significant weight in one's Enneagram. My ISTP and myself are both introverted 7s. (I know that has the potential to open a completely different can of worms - sorry for those of you who feel that this typing is impossible, just bear with me if you can.) On that ground, I feel I've been very lucky to match with someone who empathizes with and validates these attitudes and approaches toward life. It really means a lot to me that I've found this in a romantic partner.
Perhaps in the same vein, then, I love that I'm never bored with him, nor he with me. We're changeable, active, physical, mental, excitable, restless, open-minded and artistic, eager to learn new things...in love with delicious food, loud music, and different places. Travel is a mutual priority. We share the same twisted sense of humor, and we make each other laugh all the time.
I have my preexisting interests and he has his, but we've happily exposed each other to the beauty of those previously unexplored corners of life and amazingly, everything seems to fit. Even those areas we've been working in our whole lives - like photography - have benefited from our partnership. He's so gifted with the technical aspects of the craft that he's taught me several of his tips and tricks and I've become much better than I was before. He's a great mentor that way. Similarly, I've helped him become more comfortable with expressing himself emotionally. He keeps me a little more grounded; I encourage him to explore his headspace (while avoiding the unpleasant bits, of course).
I'm unable to comment on the INFJ/ISTP-couple-as-parents portion of the topic, though, and this won't change unless I meet another, more family-oriented pair with this type combination. My boyfriend and I do not want children, but we do have a very large, ridiculous cat whom we spoil with snuggles and catnip.
When It's Not Working
When it comes to our innate preferences for dealing with emotions, we do have significant differences. This is probably the most noticeable source of friction that we've encountered (on the rare occasions our feathers are ruffled at all), but it's far from an insurmountable problem. If left to fester it could kill a relationship, but I think with adequate maturity and communication skills, it can also be turned into an enormous wellspring of personal growth for both parties involved.
Though we're both pretty cool on the surface, I am the more outwardly emotional one. If there's an issue bothering me, I'm less able to hide it or distract myself indefinitely. I prefer open communication because it helps me feel better to know that the emotional climate is comfortable, that he's at ease and we can resume the complete and utter enjoyment of each other. I have an extreme dislike of negative sensation, and am susceptible to feeling suffocated by a less-than-ideal environment.
My ISTP, on the other hand, tends to play the role of the strong, silent type. He's often described as "intimidating" by those who don't know him well or have only just met him, and I blame that almost entirely upon his reticence. He's far more reserved when it comes to voicing his emotions, sometimes outright denying anything's wrong (though I'm usually able to tell otherwise) or distancing himself from the source of irritation so well that not only will he not want to talk about it, he actually succeeds in burying it for the short-term. That's not to say he's an unemotional guy - he can be quite emotional, but I had to gently work at those layers to enjoy the warmth he has to share, to experience how thoroughly playful and caring he is with me.
Having said that, I can easily see how this particular conflict of expression could make a less patient pairing fly off the rails into oblivion, fast. I think ISTPs are inclined to deal with emotions by not dealing with them, or at least by being very slow or hesitant to verbally share them, and I think respect for their space in that area is indispensible. It's a learned practice of understanding when to reach out to them and when not to, but fortunately, INFJs are gifted, intuitive communicators and can probably trust themselves to gauge the situation accurately enough. Let the ISTP go be Broody McBroodington or Tough Stuff Who Wrestles Sharks/Climbs Mountains/Builds a Better Mousetrap/LIFTS ALL OF THE WEIGHTS for a day if he or she needs to do that, and trust your gut that if they like and respect you, they'll come back soon enough. Don't sweat it too much, and whatever you do, don't pressure them, make demands, or give ultimatums. If you tell them that you're done unless they're back by eight, they're probably going to tell you off.
Moving right along, then, relationships are a two-way street (or three ways, four ways, whatever you're into). As much as I have fun being a little submissive, my ISTP and I wouldn't be where we are if he hadn't made some concessions to my needs as well. There are times when I know he feels a bit overwhelmed by my intensity, but instead of escaping immediately, he quietly puts his arms around me just to let me know he's there. That, for me, says quite a bit. If he feels compelled to go for a hike or a drive, he calmly tells me. We have an understanding that way, and as a couple, we've grown quite skilled at communicating whatever concerns we encounter. I think, as time has gone on, he's come to view those discussions as decidedly helpful tools. Furthermore, it helps that neither of us takes personal offense to anything being said; we are mature, intelligent adults and our relationship reflects that. Especially when we stay up until five in the morning playing video games in our underwear.
Since we're already on the topic, that pretty much leads me to this:
Advice for Couples
It helps to remember that ISTPs seem to be more demonstrative, physically, when it comes to emotion. Sometimes I tell mine that I love him. Sometimes he replies in kind...and sometimes he chirps at me like a bird, slaps my ass, or displays some other equally silly reaction. I'll take it, because it means he's comfortable with me and it may as well translate to "I love you, too." Gestures are ultimately a little easier for them, I think. About a month ago, mine spent a whole evening spontaneously building a new bookshelf for me, and it was awesome. Perspective is important. If you've got a less verbal ISTP who decides to change your flat tire, play an original bit of music for you, or maybe teach you proper knife-throwing technique...they're just cuddling you in a different way.
This isn't type specific, but I feel that it needs to be said, given that it may well be one of the biggest sources of satisfaction in my relationship: be on the same page when it comes to big-picture issues. If going the distance, or to have any hope of going the distance, explore your desire or lack thereof to have children (as just one example). I love my boyfriend very, very much, but I've got a general idea of how I'd like to navigate my world and children aren't in that picture; I find that for me, life's beauty and fulfillment can be found in all kinds of other ways. Don't enter a relationship thinking you can coax your partner over to your plan if it isn't theirs, too. I feel like this should be common sense, but it still occurs, often at the serious sacrifice of happiness. Know how your potential mate feels about marriage, kids, politics, religion, etc. before going rogue ninja and seducing them into being somebody else (or fighting endlessly with them for being who they are).
My boyfriend and I agree on all but religion; he's an atheist and I'm...spiritual/agnostic/mostly unconcerned. We've talked about it, and we have mutual respect for our positions without being shitty to each other. It has remained a non-issue. However, I know an awesome INFJ for whom religion is a very important part of her life; she would almost certainly not be interested in somebody like my ISTP, nor he in her. So take the big-picture stuff into consideration when it gets serious - or assess it earlier, if you're not interested in taking things as they come. Again, this probably goes without saying, but hey. There you go.
I also advise letting go of the N vs. S dichotomy, at least when it comes to those face-value stereotypes. INFJs can express themselves physically. ISTPs are neither stupid nor lacking depth. My ISTP and I can have conversations that last for hours, when stimulated and excited by a topic - and yes, the topic may be abstract. Sure, I tend to lean toward abstract topics, and sure, he may then expound upon their practical application, but we still connect and feel enthusiastic about shared ideas. There's seemingly a lot of bias against Sensors on the internet for that sort of thing, and I, in my early years of psychological type theory study, may have even been suckered into that myself. I think it is wisest to look beyond that, or at the very least, take it with a large grain of salt.
ISTPs do seem to have a particular attitude toward commitment, one I feel that I generally understand even if for slightly different reasons. My boyfriend is cool with it if we die together and has actually said as much, but we took our sweet time in growing closer to each other, orbiting like satellites until we collided - and in the beginning, having endured a series of terrible relationships, I was just about as stoked to commit to something serious as he was. It was kind of a mutual fear that we overcame together because we've decided that we're just that amazing. I'm interesting enough for him, he's interesting enough for me. Our relationship doesn't feel like a closed door or a removal of options; it feels like an emotional high that we're freely choosing to enjoy for as long as we like. And we really, really like it.
Oh, yeah - and the sex does not disappoint.