I'm in a relationship with an INFP, and very technically, and crudely, I 'stole' her from an INTJ. There is a lot more to the situation and our relationship than typology, but as far as typological factors consider into it, we both very much appreciate each other's Fi. My Fi is overgrown, and we're both a bit narcissistic being 4w5's, and we like the some of the 'mirror' aspect of our relationship, being able to appreciate each other for things we see in ourselves, the challenges of facing our own deficits in each other, and the potential rewards, and the comforts of having similar aesthetics (in the grand sense).
As far as I can tell, one of the big problems with her and her INTJ was that there was nobody looking outward, which meant that because of the objectifying nature of the FP/TJ pairing, they essentially built a fortress with a moat around themselves and the world for which each other was a structural element, and not allowed to be living/breathing/changing people. Now, I don't honestly think that an INFP/INTJ pairing is doomed, so much on why people get together and what keeps them that way depends on maturity, and individual idiosyncrasies.
In our case, I am somewhat obsessed with being a generally self-sufficient person, emotionally and practically, which is precisely what makes feel repulsed by the concept of a 'complimentary' INTJ pairing, and she wants the same for herself, which makes for a great Adam-Lilith pairing of equals.