This game fixes a lot of the problems of the first Guitar Hero game: mixed difficulties makes the 2 player game actually fun this time, and the co-op mode is exactly the way the game was meant to be played. The "practice" mode is useful for those of us who have trouble with four-note hammer-ons and means that you can actually improve at this game without having to sweat blood.
Unfortunately, this all comes at a price: the choice of songs is truly, truly awful. Barring a few standout tracks (Sweet Child O' Mine, for example), there are songs in here that are just no fun to play meaning that when you get into the harder difficulties, entire sections of the game become tedious, unrewarding slogs.
Which means that this game, in spite of its gameplay improvements, is actually worse than the original. Which is a shame. Perhaps the Xbox 360 version will have downloadable content - songs people actually want to play.