I've been to a few. I NEVER go to the conventions to socialize. Very awkward. I'm there for the rare records, memorabilia and such. Last year I snatched Rank on vinyl, an "Alma Matters" single, and Morrissey live at the Summer Sonic Festival 2002 dvd bootleg. The contests are a bit cheesy and I'm not one for cover bands although Alain Whyte's band Red Lightning was there last year which I enjoyed. The overall atmosphere is slightly intimidating.
You'll find your place regardless though. Just do what I do - save up a few hundred dollars, be first in line, snatch the good stuff, mumble the contest answers under your breath then leave when you've had enough. It really is worth it.