I can be prone to psychoanalyzing people, but I try to be kind of sneaky about it, because I realize this makes lots of people uncomfortable. Like in person I'll be somewhat calm/serene and seemingly just asking them questions for conversation or out of genuine interest. But at the same time I'm also logging certain facts and intuitions that can be used later. This method just seems to come somewhat naturally to me. I tend to be kind of selective when it comes to closer friendships or relationships, and as others said, I do like to feel informed before I make a decision.
My perceptions of others seem mostly intuition based, though. Or at least, my impressions are not always the easiest things to define, and sometimes seem to be lacking in apparent rational grounding. Though I still find my intuition to be my best judge of character. I'm usually the first to discover that I think ______ is kind of a horrible or worthless person, for example, though it is not always negative... just usually. Though I guess it's sort of bad to view people in that way... others often come around to my way of viewing that person, after they get to know him/her better. So most of my judgments seem to have been backed-up by others, in most circumstances. I tend to be on the cautious side when it comes to who I associate with. If I pick up unpleasant or even bad "vibes", I tend to avoid contact in the future.
So, I feel my ability to assess the personality of others is somewhat valuable. At least, I rely on it a great deal, in order to make sense of others. Though I think if someone develops more compassion and openness, they tend to realize that their intuitions are inherently limited. They're maybe a semi-accurate sketch of that person, but it's nowhere as real as the real thing. And it tends to take one's own perspective too seriously, to some degree. People with different backgrounds are going to view life differently, sometimes I think of if things had been different in my life, how different of a person I would be. Accounting for this can easily throw off our more grasping or vague intuitions about someone, causing us not to indulge them, so they can be seen as what they generally are... a form of self reflection about that person, based on personal symbols that are not necessarily apt in describing that person on any deep level. Unless they're a very shallow person. Depth is usually somewhat easy to discern. Or at least, the people severely lacking in depth seem to make it very obvious.
I think the analysis makes many people uncomfortable because they're kind of reduced to being a subject of study. A person can mercilessly employ notions and ideas to construct some personally shaded and biased model of that person. I can see this removing some of the magic. Also, it can easily be used as a defense mechanism, or can foster internal barriers or ideas leading to self-defeat.
) INTJ's are likely to do the same. I don't stalk over MySpace or Facebook because I wouldn't put personal stuff out there that just any idiot could see and there's privacy involved (on top of that, I don't disclose much about myself, so I'm 
psychoanalyzing again, sorry! 
It's like being caught stealing candy from a store. It's not actually THAT big of a deal, but for me, I just hate getting caught.
