After reading all of these glowing statements yesterday (was only 8 pages long at that point), I decided to go out and get it. Unlike everyone else, I'm unhappy with the game.
I had a list of small items that combined to ruin the fun for me, but decided to first point out title of this game as it is very appropriate. The Beatles: Rockband. This product, first and foremost, is about The Beatles. The game (Rockband portion) seems to come second. I would much prefer to export the songs to RB2 and play there.
Biggest issues is in the gameplay. Something has changed with the laneways (different angle? slower note speed?). Whatever it is, the notes seems to blob together (I played 2 chapters on expert guitar). Add to that the pastels with soft edges. Really didn't enjoy the sections with tight notes since I couldn't distinguish them apart.
TL;DR: I bought this game to have fun playing The Beatles tunes in Rockband. It delivered on all elements except "fun".