Interesting, as I was recently trying to divy up my mp3s by genre, and was faced with the question of what prog rock is exactly.
I do like it, mostly older groups, as my musical expansion is going in other directions at the moment. I'm a big fan of Genesis and Peter Gabriel, Pink Floyd and Captain Beefheart.
If you want to call Frank Zappa and David Bowie prog rockers, they're great too.