Identity crisis!

I took Jungian Psychology as one of my first college courses, way back in the early nineties. I loved the class, I found Man and His Symbols to be amazingly enlightning for such a dry read.
However, I am left to wonder about the effectiveness of personality typing. According to this Jung-Myers-Briggs based test, I am an ISTP. I look at what it has to say, and a large majority, almost all of it, seems to fit. Now, I also did the Myers-Briggs test in my 3rd set of courses last year, and they said I was an INTP, which seemed to fit at the time. Who to beleive? I feel a strong cofee with a Very large shot of Amerreto coming on...