I went to the GABF in 2007 and had a great time. Granted, being there when they opened up some Utopias for sampling might have swayed my opinion a bit.
While it is very crowded, you can count on sampling anything you want with a minimal - if any - wait thanks to the vast number of breweries on hand. I was truly shocked and impressed with how easy it was to move around and get samples with all of the people that were there.
The GABF was inundated with students from the University of Colorado on both Friday night & Saturday night, but the Thursday and Saturday afternoon sessions were more of the typical home brewers & beer enthusiasts.
One final note on the GABF is that some of the more popular smaller brewers can run out of their favorites. So, try to hit things like New Glarus and others like them on Thursday night and save the big brewers like Pabst & Boston Beer Company for Saturday night.
On Denver, I travel there for business a number of times per year, and I absolutely love the town. I have never had a problem with air pollution, or anything else. If you are going to the GABF, try to stay downtown so you can walk to the convention center, as well as the bars/pubs/restaurants on 16th Street (Rock Bottom), and in LoDo (Wynkoop & Flying Dog).

I've been to GABF 11 or 12 times. Denver alone is worth the trip. The GABF is the lingering frothy foam on your stout. Big, yes, but well-organized and well-prepared. I've been to GBBF (London), Oregon, Vermont and many other fests. GABF is the motherlode. Every serious beer lover should go at least once.