Saw Guitar Hero 2 tonight. Experienced it, really. Very loud.
It's pretty much the same game, as expected. The most obvious and nicest improvement is a two-player coop mode with independent difficulty levels. Their scoring hasn't improved any, though. Still doing flat DDR scoring, which I always thought was shabby. Others have agreed.
Most of the chatter indicated that there were some pretty nice additional features. Apparently, pull-offs and hammer-downs are slightly easier, there's a slow-down mode for learning hard solos, and the game's difficulty doesn't stop at merely "devilish" but continues right on to "plain evil". Apparently, there's also a lot more female guitarists.
Also, to my distinct displeasure, they continued the overamped guitar trend. In my opinion, that is a bad choice, because it really diminishes the game's party appeal. All you can hear is the lead (and, in GH2, bass) guitar. Really, really, loud. However! However! I asked the project lead, and he said he was pretty sure you could tweak the amping to normal levels, which makes me happy.
He also looked at me like a complete loon for asking it...
What I would really like for a game like this is guitar-dependant sounds. Many guitars have distinct sounds to them, and some musicians can tell the difference between two different models of Gibson. "1962... a good year." Even the most plebean, tone-deaf person will hear the difference if you substitute a banjo for an electric guitar.
I think that would be cool, if your pick of guitar actually affected the sound of your part of the song. This also means that you are likely to pick specific guitars for specific songs. Obviously, sometimes you want something a bit more razzy, death-acid style, and other times you want a more mellow, solid rock sound.
Sure, it's one of those silly hardcore things. If you could get the rights, the next and more impressive step would be to have an audio track for each singer singing each song. That would really, really rock.
That's just talk, though. Looks like Guitar Hero 2 is to Guitar Hero as what We Love Katamari was to Katamari Damacy. That's a good thing.
And, damn, they have a hell of a lot of fanboys... the crowd was 3x normal.