I managed to pick up a copy of the first Rock Band for a tenner and was immediately hooked, having already played through both Guitar Hero and it's sequel years earlier. Once I heard that you could export the Rock Band songs and any you'd downloaded to the second one, I went about trying to find a copy. Once I finally did find a copy, I started playing and pretty much haven't had much of a break!
These games outweigh the Guitar Hero franchise in every aspect, in my opinion. The gameplay is incredibly addictive and social, the graphics are great (not groundbreaking but then it's a music rhythm game, not Flight Simulator) and everything gets ten times better once you manage to get a whole band together with your friends.
The instruments Harmonix released for this game are far superior than those made for the original game. They're a lot sturdier and less prone to breaking due to being overplayed (which if anything I think is a testament to how fun these games are to play). I managed to get a cheap copy of The Beatles: Rock Band with the Hofner Violin Bass controller and it looks pretty darn stylish.
There's seriously nothing better than getting your mates round and cranking this up, especially if you buy Rush's 'Moving Pictures' album off the store. That's a brilliant album to play on drums!
This is absolutely party game perfection, can't wait for the third one.