I love the lego games. I love the rock band games and am useless at them. One thing that really annoys me is how difficult it can be to get some of the achievements, many of which require playing at expert difficulty - can't do that.
Lego Rock Band is different in that the achievements are accessible, the graphics are unusual for this type of game and are excellent fun and the track list is nice and gentle compared to the likes of GH Metallica but all great songs too. Brilliant for kids, great for adults. You can export the songs to Rock Band 2 to extend your library of tracks. This is definitely more a family game and it doesn't hurt the game at all.
Far better than Beatles Rock Band, this is highly recommended for family fun. if you're a hard-core rocker, you'd probably best miss it though.