I finally had a chance to try the demo this afternoon (I went to a Best Buy last night and their demo was broken
). I tried both the guitar and the drums, and I have to say that I like playing the drums at least as much as the guitar. I played "Tom Sawyer" on guitar and "In Bloom" on drums -- the guitar felt more solid to me than the ones that come with Guitar Hero, and the second set of buttons at the bottom of the neck are a nice addition (though I never actually used them). The drums are very accurate and seem well made too. However, the drum parts of the songs don't seem to be as hard as the guitar parts; I scored 97% in Expert mode on my first try, and this is coming from someone who struggles to pass the first couple of tiers of Guitar Hero III on expert. Of course, this could just be the song I played, since the drum playing in "In Bloom" isn't all that complex.
On top of all of that, the overall presentation of the game is *much* nicer than Guitar Hero and feels more like watching a concert on TV -- constantly changing camera angles, a realistic looking crowd... the whole thing oozes style. The interface doesn't even feel particularly cluttered, even with a person playing each of the 3 instruments at the demo kiosk.
Overall, I think that Rock Band runs circles around Guitar Hero 3 -- better songs, better instruments, much better presentation (not even including the awesome-sounding Career mode, which the demo doesn't have). I'm glad I've already pre-ordered, but now I just need to find 3 more people to round out my band...