I just spent an evening playing Rock Band 3 and using the keyboard. I've been a long-time Rock Band fan since version 1, and I generally love playing guitar but will dabble with drums when someone else wants a break.
This peripheral really does change the way I view Rock Band. It's another step (after their introduction of the drums) towards genuinely playing an instrument, and I believe could be of real value in learning the keyboard - but it's also great fun!
It's not particularly big so won't take up lots of room alongside the fake guitar and plastic drums. You can sling it round your neck and play as a "keytar" but I preferred to sit and play it on a small table.
The keyboard has a pleasing heft, and the build quality feels rather better than the drums for example. The keys have really a nice action and apparently they're velocity sensitive; not sure that makes a difference in the game but there's a MIDI port so I guess you could plug into various devices including a PC for genuine synthesiser duties. The "good value" bit from the title? Well, a half-decent MIDI keyboard isn't cheap.
In use the game gives you various difficulty settings (oh, and RB3 lets you pause, change difficulty, and restart with a well-thought out short rewind of the section in order that you & your bandmates don't lose the place). The overdrive button is set to the left of the keys, and next to that is a touch-sensitive slider for whammy-ing or other effects. The only advice I'd offer is to always use pro mode. Try the basic mode and you'll see why, but you feel like you're properly playing in pro mode.
I've been looking forward to the keyboard peripheral for ages now, and I'm very impressed. Well worth it and I can see my trusty guitar gathering dust while the keyboard takes the limelight on Rock Band sessions.