I like it. I started with the original Guitar Hero, now I play Rock Band. I don't like what Activision has done with the Guitar Hero franchise.
I'm to the point where playing the Guitar part of these games is sort of boring. It's fun if I just want to relax and enjoy music in a semi-interactive way. I have a real bass guitar in the corner that I hate ignoring for a plastic variety.
But that's guitar, I love playing the drums though. I used to play drums, and I don't have a real kit anymore or the space or the freedom to bang on them and not annoy those around me... so Rock Band drums is a great compromise. It satisfies a primal need to feel and work with Rhythm... it's not real drums but it's close enough. And the playing technique isn't real... but the rhythms themselves are real... and that's pretty cool.
And when you have a bunch of like-minded people around, these games are amazingly fun.