So I got to play this for Wii. Pretty sweet, being able to play wireless.
Anyways, the kid I babysit for recently got it, his family plays sometimes, and he and his mother both beat it on easy, but really struggled with medium.
I came over and beat it on medium - being a bass player, the earlier songs were very easy, but the later ones were much harder than I remember Medium difficulty on GH1 or 2.
The new features are pretty sick. There are Guitar Battles where you battle with the likes of Tom Morello, Slash, and the Devil (A.K.A. Lou). Playing a metal version of Devil Went Down to Georgia is pretty awesome.
I'd have to say these are probably the best selection of GH songs in any of the games I've seen so far. A couple of cheesy ones, but overall great music. Sunshine of Your Love, Knights of Cydonia, Barracuda, and One are the first few that come to mind - Through the Fire and the Flames is a bonus track that you can play along with during the credits and is STUPID hard on medium.
Anyways, just wanted to put my two cents up and see if anyone else had played it yet and wanted to share some thoughts.