Harmonix revitalized the music/rhythm genre with a little game called Guitar Hero. Now they have taken the next step with Rock Band. While Neversoft played it safe with Guitar Hero 3, barely changing the time tested formula, Harmonix took the extra step and added drums and singing to the mix.
The result is an incredible co-op and solo experience. Having the ability to play with a full band is incredibly fun. At the same time, with three instruments to master, you will have a lot to do by yourself as well. The Band World Tour finally did away with the boring list of songs that Guitar Hero 1-3 used to deliver a much more interesting game, even if it is limited to co-op.
One of the best things about this game wasn't even known upon release, that Harmonix would be releasing weekly DLC, already there's close to 200 songs available for RB, including the ones on disc. This gives the game an enormous replay value. And since all these songs will also work with the sequel, there's no reason not to get any.
Of course there are a few flaws. The solo career is still just a list of songs, the instruments aren't as sturdy as their Red Octane counterparts, and the online play isn't excellent. But the sheer awesomeness of being able to play as a four piece band and the incredibly replay value because of DLC more than makes up for these small faults.