Between the Forer Effect ( http://en.wikipedia.org/wiki/Forer_effect ), confirmation bias ( http://en.wikipedia.org/wiki/Confirmation_bias ), and the fact that people routinely retest into different categories ( http://en.wikipedia.org/wiki/Myers-Briggs#Study_of_scoring_c... ), I don't place much stock in Myers-Briggs. Add to that the dubious way in which the theory was developed, and I'm completely mystified by the…
as expected, people here are overwhelmingly INTJ/INTP. you'll find that's the case in most scientific disciplines
it's not easy to see how the theory is good without having noticed how particularly similar people can be
these type combinations are not just 'any four categories', their effect on a type is greater than the sum of the parts. a bit more descriptive is the underlying Jungian functions. for example, INTJ is syntactic sugar for Ni-Te, introverted intution primary (the part that makes them so confident) and extraverted thinking secondary (the part that makes them logical). INTP is sugar for Ti-Ne, introverted thinking primary (the part that makes them absent-minded), and extroverted intuition secondary (the part that makes them look at the world for answers)
so by flipping the J to a P, the whole type is different. not all letter flips are that significant. I/E is the closest. for example, Feynman (ENTP) and Einstein (INTP) were Ne-Ti and Ti-Ne respectively. Feynman was driven by extroverted intuition, while it served Einstein. the nature of their discoveries is similarly 'playful' and creative
also, these four categories are not poorly-chosen. the most significant category is the S-N axis. it's basically this simple: if you meet someone who shares this same letter (you are both S or both N,) you will click. otherwise you won't
here's some forums where the type differences stand out
http://infp.globalchatter.com/messageboard/ -- fluffy
http://www.intpcentral.com/ -- levitous
http://intjforum.com/ -- dry
one thing to keep in mind is these aren't 16 "boxes." it's a bit complicated. for example, i suspect pg is INTP, but not standard-issue. he might be an Enneagram 3 INTP, for lack of a formal label. it's something he would share with Maddox ^_^