With a huge amount of DLC, Rock Band 2 easily rocks the socks off the competition.
The game does not break the boundaries of the music genre like its' predecessor did but it is a worthy addition to the series. The introduction of a 'Battle of the Bands' mode is a good step and the amount of customisation is very impressive. More could have been done with the online however, 4v4 like Guitar Hero World Tour would have made the game better but Battle of the Bands is a decent replacement anyway.
One thing I especially love about this game is the random way the songs are chosen online in ranked Score Duel and Tug of War modes. It stops players from constantly picking the same song while incorporating any DLC that both players have. The online band mode is very impressive too and is a welcome addition that was lacking in the first Rock Band.
The difficulty of the songs in this game is not as tough as Hot For Teacher and Satch Boogie in GH: WT but they're secret weapon is the Rock Band Music Store. There are tons of songs for people to download, suiting all tastes and difficulties. I suggest if you are an expert guitar player, try Yngwie Malmsteens' Rising Force Pack. Capri Di Diablo could be one of the hardest songs ever made for a music game (at least if you play guitar it is).
To sum up, this game could have done with something a bit extra after Rock Band 1, but the game's real strength lies in its' DLC.