I recently read this preview of the new, more realistic, Rock Band 3 instrument controllers. The improved guitar controller looks like an actual guitar, because it is, and the new keyboard controller looks like a mini keyboard. If you play in pro mode, you have to hit the right fret and string for the guitar or the right keys on the keyboard, while the lower modes are the same as always. The controllers seem to trade off accessibility for realism, and I like it.
I played Guitar Hero a few times and was never very good at it, I imagine I'd be worse with the new controllers. With the accurate frets on the guitar, there are 102 buttons as opposed to five. Guitar Hero always seemed accessible, you can pick up the controller and play for a bit without too steep of a learning curve. How can you pick up a controller with 102 buttons and not feel a little intimidated? Rock Band was always a good party game, but what kind of party game takes 10 minutes to learn?
When playing Guitar Hero, I almost felt like I was playing a real guitar, with the new controller, I actually would be. Instead of just using a gimmicky controller, the new controller is serious. With Rock Band 3, you can actually learn how to play the guitar, which is awesome. In fact, you can replace the controller with a real MIDI-compatible guitar, if you have the adapter. You can also use the controllers in music editing programs via the MIDI ports. You can still play the old way, with the old controllers if you want, but I can't imagine choosing that over the new controller. Chances are that your Rock Band skills won't transfer over to a real guitar exactly, but you'll get close. Plus, Rock Band is way cheaper than a real guitar and lessons.
I would play Rock Band 3 if it could teach me how to play a real instrument. I like the idea that after a few hours with Rock Band, I could play a real guitar, not well of course, but I could still play it. What the controllers lose in accessibility, I think they make up for by offering the possibility of learning an instrument.