I've owned this game for over a month now and I suppose I was saving it for the holiday break. Even though I did a ton of gaming over this holiday vacation, for some reason I never broke out Lego Rock Band until my New Year's Eve party. We ended up playing after midnight, so this was officially the first game of the new year for me. Unfortunately, I was drinking with friends when we started playing this, so I didn't really get too far into the game. We played mostly quick play and as far as I can tell, the achievements are all centered around the story. I'll have to return after I've played around with that a little bit.
Update: January 24th, 2010
I entered story this evening, played the first couple of songs in the career. The gameplay is all Rock Band, so there's nothing new there...but the cut scenes are really funny. You're a little Lego dude, and instead of outfits, you can unlock interchangeable body pieces, that look just like the Lego mini-fig toys. The songs in this one seem targeted more towards a general audience...and that's cool with me. It's not like I don't have 300 rock songs waiting for me in Rock Band 2, and if you're feeling especially lazy,all of the songs from this game can be exported to that game for a mere 10 bucks. Not bad.
I am probably going to shelve this one until the next time I have a bunch of people over. Rock Band is best when played that way.
Overall rating: It's more Rock Band, it plays exactly the same as the other Rock Band games, so it gets an 8/10 from me.