I got to play a good amount of this game on PS3 on Thanksgiving. My sister's ultra-rich in-law's had their offspring playing it. I did ok on medium, and got to play the bassline to Suck My Kiss. That was fun.
I was watching two people play co-op, and we only had one guitar controller. He was on bass, and since he sucked he was playing on easy. I wanted to play it again but on a harder difficulty but it had been an encore, so I don't know if you can do that in free play or whatever it's called.
I just recently got it for the Wii. This is the first GH game I've played, but I really like it so far. Some songs on the setlist are better than others, but it's definitely fun to play.