The best way to describe Guitar Hero II, if you've played the first game, is to say that everything's better except the songs.
I've beaten the game on Hard, which is right at my sweet spot for Guitar Hero--on Medium I play notes when I'm not supposed to, and Expert is so hardcore that you'd really be better off learning to play the instrument itself. Hard is close enough that I can enjoy the illusion of actually playing along without too much stress. And Harmonix has done a good job of addressing the little frustrations of the first game, with easier hammer-ons and pull-offs (they were originally unrealistically difficult) and a much-needed practice mode. The addition of encores is a cute touch, and the game itself looks better.
So if it weren't for the songlist, the GH2 experience would be a lot higher. But you get the feeling that all the really inspired choices were picked up for the original, and now you're sorting through the leftovers. There are moments of genius, especially "Tonight I'm Gonna Rock You Tonight," "Sweet Child O' Mine," and "Jessica." On the other hand, "Killing in the Name" is a monstrosity, and "Institutionalized" a disaster. It's a much more uneven playlist. The other song criticism is the production--sometimes the other parts of the band are very soft and difficult to hear--although that could be my sub-discount TV at fault.
None of this stops Guitar Hero II from being one of the best games of the year practically by default. You still can't go wrong with this game, especially as party entertainment or for aspiring musicians. I can tell that I'll definitely keep playing through it just for the experience, as I did with its addictive predecessor. If I had to choose this game or the original, it'd be a tough call--beginners might want to start with GH2 for the responsiveness and training options, and graduate to the original when they really want to bring the Rock. And the sooner Harmonix decides to use this improved engine for more selective themed or genre song packs, possibly including songs from the first game, the better.