Got the game the day it came out and have had much fun with it since. I live with 2 other guys so obviously it's great for a bit of multiplayer gaming.
I have had some problems though. My guitar was randomly playing notes. On investigation i realised it was the new touch sensitive bar. As i wasn't using it I decided to tackle the problem. (Warning, this will most probably invalidate the warranty on the guitar) Firstly, i took the neck off the body and unscrewed the back cover of the neck. Took out the circuitry and unsoldered the wires connected to touchpad. Put it all back together and now it's working fine (although i am unable to use the touchpad obviously). Besides this fault the guitar feels great and i personally find the new star power button very useful.
I've also had a problem with the drums. The red pad is not as responsive as the rest and to get it to register (without feeling like you're hitting it so hard you're going to snap a drumstick) a hit you have to hit it dead centre. Not the easiest thing when you're rocking through a bit of fleetwood mac! Red Octane (the makers of the peripherals) have released a fix for this. On their website you can get a free USB to midi cable (with your drum serial code). This enables you to hook the drums to your PC where you can download a program which will allow you to adjust the sensitivity of your drums. I haven't received this yet but all reports suggest that this fixes the problem.
These two issues are in my opinion major problems and are the reason that i have only given this game 3 stars. However, if you are prepared to deal with this there is a great game waiting for you. The song list in my opinion is much better than GH3. And the songs are all in gigs now. No going back to the menu to select a song, you just play the whole gig. Also, the songs are all unlocked through gigs instead of having to buy loads as bonus songs as in GH3. Free play is also better than GH3 because you can create a gig of 6 songs so you can select any 6 songs you have unlocked and play them back to back (instead of just 1 song then going back to the menu).
All in all this game is extremely fun!!!! It is just let down by poor peripherals which is unnacceptable for the cost of the game.