We picked this up a couple of weeks ago and all I have to say is wow...
The disc comes with over 80 songs and they are making another 20 free downloads available in a couple of weeks. For a $5 licensing fee, you can rip 50 of the 53 songs from RB1 to your hard drive and all of your previous DLC is fully compatible.
Right now our library is so huge, we haven't had a reason to play GHIII since we bought this.
Our favorite feature this time is your musician isn't tied to one instrument in the anymore. You can play from any position with the same character.
I highly recommend this for anybody that enjoys music games.