Most of my casual listening is done in 2.1 up in my living room open to the kitchen and dining room. Unless I'm sitting back on the couch reading a book, I'm rarely in a position to benefit from surrounds.
However, in my basement TV room I have grown to really enjoy DLP IIx for the extra envelopment that 5.1 gives you. Definitely most noticeable on classical as others have mentioned but I find myself turning it on more and more for other music varieties. I really enjoy DVD concerts, even if I'm not watching the video and since they have all 5.1 channels built in (some better than others,) I definitely use 5.1 for them.
Lately I've been playing Rockband on PS3. The music quality for the songs is actually quiet good and it's 5.1. The cheers (or boos) of the crowd all around you while you pound away at the drums.... Now that's an enveloping experience. heh heh.
With great power comes Awesome irresponsibility.