This really is top if it's class, definitely one of the most genuinely fun games I have played in a long time. The gameplay is very user-friendly and all looks very attractive. The range of songs available so far is staggering, I'm closing in on the 100 song mark already and we're promised a 500 strong catalogue by the end of the year!
I own Guitar Hero 3 and thought that was a fantastic game, but Rock Band really is a step up, it looks a lot prettier, is a lot easier to use and is generally easier to play. Agreed Guitar Hero is a lot more challenging, but Rock Band is a more family-friendly game and can actually be done on expert without too much difficulty!
The multiplayer is absolutely great fun with a bunch of friends and the world tour mode is brilliant. You can completely customize your band and rock out like there's no tomorrow! Even when you're on your own you can stil play as a band online with new people which is a great feature. The single player is great as well and a great way to practice on the different instruments.
I strongly recommend this game to anyone, it really is a lot of fun and there are some great songs to play.