My sense is that it's a useful model for some things, but I'm most interested in how useful it is for relationships.

I think there are better models to use when considering relationships. I note that often such models are useful in as much as they serve to provide a language which can be used to describe intuitive associations that we pick up through observation. The model is not terrible, being a formalisation of the 'opposites attract' conventional wisdom with consideration given to how different people relate on intellectual and emotional levels.

As for MBTI, I have found it useful in some regards. I know, for example, that I can basically rule out relationships with anyone who comes in as a "J". I just find "J"s annoying ('judgemental' of me, I know!)

Edit: The links you provide are... interesting. I must admit I have rather strong doubts about just how accurate those physical descriptions of various personality types are!