I have to say that I was sceptical about this game: I do actually play guitar and I figured that this wouldn't be my cup of tea at all. However, one of my mates borrowed it off a relative who bought it in the States and we all trooped round for a crack.
Initially, I felt a bit self conscious with the singing and I found the guitar very unnatural and I imagine most guitarists will feel the same to start with. The drums too are a bit odd as I used to play those too and the normal style of play is to hit the high hat with the right hand and the snare with the left, the right hand being above and crossing over the left. In Rock Band the layout means you have to play the snare with your right and high hat with the left. It wasn't looking good at this point.
Thing is though, after about five minutes, I just didn't care about that because they're not really instruments, they're game controllers and the sooner you remember that the better. Sounds obvious but until you rememebr that cars and golf clubs aren't controlled by little rubberised joysticks in real life, you'll be annoyed by the game if you play instruments. Once you do get over that, God it's good fun. Really good fun. You find yourself really getting into it and after songs that go well you start high fiving or knuckle tapping your band mates and saying "dude" a lot.
Obviously, your band name is important. Try and make it funny because one of the best laughs of the game is seeing all the other band names and giggling like schoolkids at the innuendo. Time will fly by and before you know it, it will be late in the night and you will actually be quite tired because you really do start to give it your all without realising it.
So to summarise, this game really is damn good fun even if you think it won't be for you, give it a go if you get the chance as you might just be surprised. The only down side is the cost. In the States, the game, drum kit, microphone and one wireless guitar will set you back about $160-180 (soemthing like £80-90 or so) but over here you are likely to pay the dollar price in pounds for the same kit which is pretty scandalous as I'm damn sure that soemone is getting a scorching deal and someone is getting stung, and I don't think it's gamers in the States who are getting the bum deal. Just think of it this way though: this kit allows a bare minimum of three players so split the cost with mates and it's not so bad. I think you can use Guitar Hero controllers too so if you have one already that helps. Only thing is, you really need another guitar (for the bass) and a very decent size telly so you can see what you need to be doing.