I used to goto them. They cna be kind of neat, but I can't stand the crowds, and the prices generally aren't very good either. They used to be better back before fansubs were on the net, because a lot of times it was your first chance to see a new anime. I haven't been to a convention in a couple years now, the last one I went to was fanime 2003 I think. I was also at anime expo (west coast version, since there is an anime expo new york now I think) in 99, 01, and 02.
I don't foresee myself ever going to another one though.