My ex boyfriend of 6 years was an ISFP. Provided, he was also diagnosed bi-polar, so some of the differences could have been due to his illness.
We had an INSTANT connection the day we met. Since we didn't exchange numbers that day, he actually contacted a mutual friend for my information because he "had to see me again right away". I should note the SAME exact thing happened with a later boyfriend who I believe to also be ISFP.
The beginning of the relationship was a LOT of fun. We were constantly doing some sort of activity - usually outdoors related because we were both passionate about hiking & camping. He was really good at coming up with new & exciting things to do. When I was in his "moment", he might as well have been my prince charming. Both of us fell hard and fast for one another - I think we sad "I love you" within a couple weeks of dating. After the initial year or so, we started running into problems though.
I think it's the Fi vs Fe thing. I've always had a strong Fe, to the point where I've questioned whether I was an ENFJ or an INFJ because my Fe & Ni are pretty much in a dead heat as far as usage. He had a strong Fi. Our relationship turned into it being all about HIM - he was very much inside his head in a Fi perspective and I was taking care of him & attending to his needs in a Fe sort of way. It turned into a very unhealthy situation for both of us because I was essentially enabling his depression and making his life my life without getting any of my needs met in return. Had I been a bit older and wiser, I probably could've caught myself before getting so absorbed in his inner demons, but it was my first serious relationship & I guess I didn't know any better.
Eventually, the relationship was basically based on him - his moods, values, etc. He was very sensitive to criticism, but felt the need to criticize me. There was cheating involved on his part, but even without that it was a very unhealthy relationship.
Anyway, I'm in no way saying that ISFP & INFJ doesn't work - I dated an emotionally unhealthy ISFP! I just wanted to share my experience.