Solid sequal, but the setlist lacks flair.
Some new things I like about GH III:
1) I like the new layout of the basic gameboard. A noticeable addition is a counter that keeps track of your current note streak.
2) The boss battles are cool. They add a new aspect to the game that is both challenging and fun.
3) The graphics got a huge overhaul, and the characters are much more finely detailed. All around, it's a much prettier game.
Some things I don't like:
1) I've got to complain about the setlist. There are quite a few very cool songs in the game, but having played all of the regular (non-purchased) songs, I can come to no other conclusion that the setlist from GH II was much stronger.
2) I don't care for the new whammy bar animation. It's hard to tell how much modulation you're getting from it because the waves created by it on the screen are difficult to see.
3) The graphics overhaul perhaps went a bit too far. I've noticed several instances of graphics slowdowns, especially when I hit the star power. For a game that relies so heavily on timing, this can really cramp your style.
I don't have any regrets about purchasing the game, but I don't think GH III lived up to the bar set by its predecessor.