Fun SP or MP...but boring career mode and it's very expensive for what it is.
Well it's RockBand really what else is there to say. It's fun playing with mates or online and great for actually learning to play the drums. It's incredibly challenging on the higher levels but there's too big a jump in difficulty between easy and medium noticable when you do have many mates around who have never played.
The only big disappointment is the career mode. You get to create a band and play certain gigs to unlock songs and other levels but it's just boring... there's no real goals or targets to achieve on each gig. To pass you just avoid getting booed off stage which is pretty easy. The manager option you have is a waste of time as they barely make any difference. and generally you don't get a feeling of being a poor band working your way to the top playing to a stadium of 80,000+. Also there's also no real incentive to move up a level difficulty wise, they should base how good your band is by what difficulty you're on with newspaper/fan reports on how good the band is compared to other real life bands.
RB3 will need to add more than just extra tunes to keep myself and probably many others interested. A proper management mode will be awesome with some GuitarHero style face off with other bands and rewards for high difficulty levels and albums deals etc.