Benefits:
1) I love the fact that we're both introverts... there's something reassuring in the fact that we don't mind having alone time to ourselves or alone time with each other.
2) I admire his logic; he admires my feelings. I love it when he takes the time to explain to me whatever he's thinking about/working on. He can get so hyped-up from the intricacies of a subject, that I'll get swept up in it, too. It doesn't happen too often, mainly because he is so quiet and usually lets me do the talking when I get into "intuitive, emotional rambling mode."
3) I absolutely love to cook and take care of him in ways that are immediate and physical. He caters to my nurturing side and I cater to his sensing side. Added bonus - in return he usually outpours some romantic mushy-type feelings that he's been keeping to himself.
His friends, I'm sure, are quite surprised by our interaction - the first thing they usually tell me is "you're perfect for him" and the next thing is "I've never heard him say things like that/I've never seen him do things like that."
4) I don't know if this is the case with all ISTP's, but I know with him he can be extremely negative about other people, the world, his life, etc etc. To him, he is viewing everything through a hyper-realistic point of view, and he really embraces the idea of the life being this chaotic, strange, and meaningless experience. I'm the exact opposite - I can be way too idealistic, hopeful, and ascribe too much meaning to everything. I enjoy the idea that things all are interconnected and meaningful.
Whenever he gets into a rut, I usually lift him out with my overly idealistic nature - and when I get into mania-mode, he can usually ground me with his realist nature. The same applies to people - when he gets frustrated with people, I just remind him that they're all people and they all deserve a chance. When I get frustrated with people (from spreading myself too thin), he reminds me that not everyone is worth a 110% effort on my part, because some people are just assholes.
Problems:
1) When he gets frustrated with something, he either wants one of two things: for me to leave him alone and give him space, or for me to jolt him in some sudden way to "snap out of it." It's hard for me, an INFJ, to dish out advice like that. I usually want to sit down and dissect and converse the problem out, which just makes him more frustrated because he feels like it isn't getting resolved. Nowadays, when he feels stressed about doing something, I just snap and tell him, "just do it! get it over with!" and he happily proceeds.
2) Of course, his coping mechanisms don't work with me as well... especially when I'm upset about something. There have been so many times when he's told me, "Just stop worrying" or "Don't be angry." I understand that he pointing out my over-feeling/over-worrying side, and that, really, the only solution is to just stop it since I'm doing it to myself - but occasionally it gets frustrating to hear because I just can't help it in that moment.
3) His logic is always right/My feelings are always right. We both depend on them too much. When they clash - forget about it! I can't tell you how many unresolved arguments we've had on those grounds. But, we can usually just accept the fact that we'll never really agree - and then later, after I have the time to think his logic through and he has the time to absorb my feelings, we come to a compromise without even talking about it. It's strange, but wonderful.
4) His hobbies seem overly dangerous/frightening/physical for me sometimes. My hobbies seem cautious/boring/too mentally involved for him sometimes. I'm okay with him not liking my hobbies, since I don't mind doing them alone. However, he usually wants me to do his hobbies with him... I've tried to be open to it, since I enjoy that impulsive side of him, but I just don't know how okay I am with snowboarding or sky diving. =X