I haven't been that impressed with the song list so far. I haven't played through all the songs yet...just the first 25 or so, so there's still plenty for me to unlock, but I think I like the songs on the first Guitar Hero better for the most part.
What would be cool is if some one with a big music library, like Sony, made their own version of Guitar Hero, then offered a huge chunk of their catalog for download at $1 a song or something. I'd buy a few hundred songs.
Edit: I played through easy mode and 2/3rd through Medium mode. The encore songs are all pretty good. The songs you unlock still suck, just like in the first Guitar Hero.