Tough one to rate - the game itself is great for the most part, but the instruments are extremely cheap and unreliable..
Harmonix knows rhythm games. They know how to make note charts like no one else, in my opinion. That's a huge plus for them. HOWEVER...
Harmonix does not know hardware. Prior to their split from activision (for whatever reasons, however it went down, it's irrelevant), they had RedOctane making the controllers. My Guitar Hero 1 guitar lasted me through 4 Guitar Hero games, and Rock Band for PS2.
Having sold my PS2 and upgraded to a PS3, I sold the PS2 guitar and all my GH games, and picked up Rock Band so I could see what all the fuss was about with the DLC and online play features (missing from the PS2 version of course). The drums were the only thing that was compatible (I didn't own a microphone), so I bought a PS3 wireless official Rock Band guitar as well.
Within 3 weeks time, I had gone through 3 guitars. First one, Tilt Sensor was all over the place. Sometimes it would just randomly go off. Most of the time it wouldn't respond at all. Not a HUGE deal in solo play, but in competition and online play, a much bigger problem.
So I returned that guitar, only to get one that wouldn't even power on out of the box. Another 30 minute drive to the game store (Same day), and guitar number 3 finally works it seems.... until after 2 hours of play, the tilt sensor starts not recognizing a full 90 degree upward tilt all of the time (missing about 3-4 out of 10).
Amidst this, broken drums (my friend has gone through 2 sets of drums already, and 2 guitars himself), and some missing features for the online play, I cannot see how any reviews could give this game a 9 out of 10.
ONLINE PLAY: It's a lot of fun to get together with friends online and tour as a virtual band. Pick up bands though, are hit and miss as with any online "Random" matchmaking system.
The real problem with online play, for me at least, comes into the Tug of War and Score competitions. While playing both of those modes, 2 days in a row, I had 9 people quit after they lost one round. One "girl" I played against, won against me twice, I was ahead in the third round, she bailed halfway through the song. The others, quit right in the first song if they weren't winning right off the bat. THIS IS VERY FRUSTRATING AND REALLY RUINS THE FUN OF THE PICK-UP ONLINE PLAY. Only 2 people I played against, continued playing after losing, one of them was a great match for me (we were neck and neck constantly), and - WHAT REALLY IMPRESSED ME - an 8 year old played against me, lost 6 times in a row, but had a good time, and was the most polite player I had ever met, and I would be glad to play band mode or any mode with him anytime. Wish more people could take a cue from this 8 year old kid.
Another problem is that if you want to do say a guitar tug of war against someone, you can't choose to be the leader, choose the song and difficulty in advance, before someone joins. Likewise, you have no idea what level of player you are going to join in against. I was seriously mismatched against players that were Easy/Medium difficulty while I was doing Hard and Expert. There is no way to just set up a set list either for those online modes, instead you must select each song individually after the previous finishes. These are things I believe Harmonix could have fixed with patches or updates, but unfortunately haven't.
Another issue, is invites for bands. I had a PSN "Friend" inviting me to join a band TWICE, and it never worked. the first time, I was in-game, and never got any notification of it that I could tell. The second time he sent an invite, I again didn't know about it until he was in the process of exiting the game. This is another area that is seriously flawed, and hampers the enjoyment of online play.
So, to summarize, here we go:
1) Faulty instruments that don't hold up well. Buyer Beware - 60 dollar guitar has only 60 day warranty through EA Games. They aren't cheaply priced, but yet they aren't reliable for the most part.
2) Online features needed more thought/work to foster a better community
3) Tons of DLC makes it hard to get bored with the songs, but that also comes at a price... of 2 dollars per song (Way overpriced imo, it's not like you can transfer it to PC or ipod and actually listen to the track as well, so to me that is a total waste. Full albums retail for 14.99, and again, they are good for Rock Band, and that's it. Way too much.)
4) Great Note Charts make the game fun to play at any difficulty (although, I highly recommend to start with HARD, as this will get you started using correct finger placement earlier on)