Guitar Hero III is the best in the series, with updated graphics for the PS3 and a varied, wide-ranging song list.
The immitation guitar controller is the best one out of them all thatnks to the new whammy bar and more sensitive buttons, and the removal faceplates are a brilliant idea.
The song list that ships with the game is brilliant and has actually got me interessted in a genre of music I would have never listened to before (rock). However, don't think that this game only offeres rock music (which is what I thought before I bought it). The tracks cover all genres, and many are avilable on the store for download (at a small price or sometimes free). My only niggle now that World Tour has been released is that the release of new songs for GHIII seems to have stopped completely, and I would love to play "another way to die" which has just been released for World Tour.
Online modes are also a lot of fun, with both battle and co-op modes. You can pic either, 1, 3 or 5 songs to rock out with and the other person can either help you amass a huge score or battle against you to see who is the better player.
Overall, a thoroughly enjoyable game which is a lot of fun to play, especially with the speakers turned up!