They play the more rockin' songs early in the evening. As the evening goes on, it gets more into electronic/house kind of music. They usually mix it up a bit though. In a recent blog entry Bob said they were going to focus more on the rockin' sets.
It gets very crowded simply because of the layout of the BackBar. There's no real dance floor, so when people want to dance, they just start dancing wherever they happen to be standing.
I haven't been in a while. It's lots of fun. Sir HC is correct though - It can get very loud.