I am fond of my INTP. I won't lie, it is probably not the easiest choice in a relationship, but it has been meaningful and resilient. We both see each other as capable of seeing a side of any given problem the other does not see naturally. It is also fairly non-threatening way to be challenged on deeply held beliefs. We are both deeply IN x P, so even though we don't always come to the same conclusion, we kind get each other's mental processes to reach the conclusion and totally understand each other's social awkwardness. In social situations, I'm afraid I will be too honest about what I'm thinking with other parties; so is he. The main difference is I'm psychoanalyzing all of the other person's issues, and he's considering string theory. If either of us were to share our thoughts with the other person, to each of us, the other person would say "man, you overthink things to much". Or they would cry.
INFPs and INTPs have very similar ways of being in the world. We're deeply, deeply thoughtful, deeply reserved, and yet want to be known and loved for our authentic selves.
On a more critical note, I think INTPS and INFPS both have the tendancy to be space cadets - INTPS like Albert Einstein and INFPS like Aldous Huxley. I think we both favor the ethical good, we're both out of the box thinkers, and we both value independence. For the most part, respect of each other's eccentricity and independence makes living together quite comfortable. Where it gets difficult is the INFPS constant quest for romance; INTPs are the kind that will tell you once and that's it - and when they say it they actually mean it. It is a hard adjustment, but the proclamation of love is more authentic and straight-forward than most types. You just have to learn to settle for a bit less than the ideal, which, being an idealist, is like trying to give up heroin.
In this duo INFPs essentially learn to better use their brain while INTPs learn to exercise their heart.