I went to the first GABF in Boulder and attended every one of them until about four years ago.
Either I'm to old for the hassle now or maybe it has gotten so big that it's impossible to really seek out unusual and special new brews among the unbelievable numbers of beers.
But because it brings in so many brewers etc, the best part of the festival is, IMO is visiting the local craft brewery's and brew pubs tasting rooms. There are always lots of fellow beer geeks to talk with and tell lies to at these places.
Falling Rock Tap House is a kick and of course is THE hang out of choice if you get there early enough to grab a stool or booth. They always have ton of new stuff to taste and you'll probably run into the who's who of craft brewing's movers and shakers.
Going any one?
I went once a few years ago, and I'm in no rush to get back to the festival itself. Its a great experience that every beer lover should do just to see the scale of it. I feel the same about Ofest in Munich too. The one ounce samples just burn out my palate after about 20 of them, and is not very conducive to socializing since you are always on the lookout for your next sample. Love hanging out and drinking beer in Denver though - which I seem to do every other year, and this year is the year I think.