A portion of a Kotaku review...

You get half as many songs as a normal Rock Band release and no online multiplayer, in exchange for a rhythm game that's brimming with unique LEGO personality and music you won't find anywhere else, until Harmonix releases the tracks as paid DLC for the regular versions. Plus, LEGO Rock Band is by far the most accessible of the Rock Band titles, with a lower level of difficulty making it an excellent gateway game.