I've always likened the MBTI to a blood pressure test. Trying to draw a conclusion from a single incident is meaningless. Several tests, over a period of time, will provide a clearer indicator on one's personality.
Then you have to factor in that different tests might have different questions, which in turn lead to different results. Lastly, results are affected by the reason for the testing, and an awareness of
what MB tests for. If it's done as a blind test (that is, unrecognizable as the MBTI), then more objective results are possible.
I found out about this forum because of a job posting on Craigslist. In addition to resume and cover letter they wanted a copy of my MBTI from a site they linked to. It's a call center, Customer Care, which I'm
really good at

), but I know they don't want an INTP! So I answer the test differently, and sent them my ENTJ results.
For me, it isn't so much about finding a strict categorization of myself as much as it is looking at the definitions of type offered up by different testers. As I put in my profile I'm primarily ANTP, "A" being "Ambivert" (not my term). I'm not likely to join the ENTP forum, though--they just aren't as enigmatic as the "I"s.