Just because it doesn't improve on Rock Band doesn't mean it isn't a top quality game.
There is an awesome list of songs from all different genres of music and there is something probably to please everybody, unless you enjoy music without any guitar in it. If the latter is the case, do not buy this game.
The guitar has been thoroughly improved because the size increase makes it feel more real, and the microphone is very good as well.
My biggest criticism is that the game seems to be too forgiving of mistakes at a first glance and isn't as challenging as Guitar Hero II was. Besides that, the game hasn't been bad at all so far and I look foward to playing it some more.