Oh, yay! An interesting topic right upfront after being gone for a few months.
As far as, I suppose, psychological "metrics" go, I really like the MBTI. I've heard it compared to astrology and horoscopes (which for their end goals I suppose is at least somewhat fair...) but generally, if the subject understands the meanings of the dichotomies well, the final result will be pretty well consistent. It fills a purpose which other personality "tests" don't, because most focus on diagnostics and identifying psychopathological characteristics; the MMPI is more of a diagnostic tool than anything, and the Big Five, while detailed, empirically based and far more comprehensive, is better suited to large-scale data correlation than anything.
One of the most frequent criticisms of the MBTI is that it is very "black-and-white", since there are only four dichotomies, producing a total of only 16 possible personalities, with no officially built-in mechanism allowing for, say, an ambivert. I like that, actually. The fact of the matter is, unless you design a personality test with over 7 billion possible outcomes, it will always be limited. I enjoy the fact that the MBTI is limiting enough that generalities can be inferred. Yes, you will be "put into a box" to some degree, but it's varied enough (and has been around long enough, with a lot of research being done regarding it) that we now have a simple, relatively easy-to-understand, and widely applicable tool which can say many, often profound and useful things about you and other people. I don't advocate its use as as, say, an employment screening tool, but on the other hand, it can help employers understand how best one of their employees can fit into the business.
My main problem with the MBTI is the attached cognitive functions which lack empirical backing. Supposedly, socionics fixes this somehow, but I haven't looked into it too much.
Criticisms aside, though, the MBTI is just plain fun to me! Or if not fun, at least introspectively satisfying. It's a real pleasure to see MBTI-described characteristics in action in the real world, such as in the high level of NT- and P-inclined members on this forum. I could talk about MBTI stuff all day...
I'm surprised no one's posted this yet:
Oh, and if you're curious, I'm an INTP.