I went to GenCon in 2006 as a "once in a lifetime" gamer vacation. When I got back I told the wife, "when we go next year...." and I've been back every year. I met Kevin (P-Cat), Dave (Crothian), and Jody (Rodrigo) back then and have been pretty good friends with them ever since.
Unlike Dave - I think True Dungeon is good every year - of course I work for them now, so my judgment may be clouded.

I'm also the audio producer for the ENnie's - a great little affair we seem to have every year, sponsored by some web site.
But, he's right, every year it becomes more about friendships and less about the actual gaming - not that I don't game, just that I go to game with my other friends, the ones I only see once or twice a year.
If you get the chance take it. As for who shows up, it's a mixed bag: Gamers of all shades, pnp, ccg, electronic, wargamers, simulationists, and from every walk of life, race and age - from families with small children to doctors, I've seen it all. There are quite a few comic/anime enthusiasts beginning to make GenCon their home too and while it means the tone is changing a bit, it's still a gaming convention.