Good premise, but there are quite a few problems keeping it from being truly great.
Introduces gamers to the goods and greats of the oldies; the soundtrack is composed primarily of classic rock, which is something that gamers need to listen to.
Fun! This game has some great pick up and play value.
Great party game, if you can actually get two Wiitars.
Infinite replayability: The saving grace of GHIII. If someone gets truly addicted, this can be a great game.
The bad:
Time-waster. Finishing the game does not feel like an accomplishment, and neither does completing a song. I know that games are supposed to be a waste of time, but when something that involves no strategy actually makes it feel like you're wasting your day, it's not worth playing.
Expensive. The WiiTar might be pretty, but $90 for the game and controller is insane.
Advertising-packed. Granted, it's for guitar companies, but it's still incredibly annoying.
Too challenging: Repeating a song over and over again to learn it is not fun. Yes, I know that's how you do it on real guitar, but Guitar Hero is not real guitar, it's a game.