It’s worth mentioning that for my keyboard-playing in Rock Band 3, I’m using the official keyboard controller, and I’m really quite impressed by it — it’s solid, the keys feel great, and it has a standard MIDI output for use with a PC or other MIDI gear, so for the money (AU$135-ish locally, US$80 on Amazon), I think it’s a bargain. There’s a Rock Band 3 MIDI interface coming, which I could’ve used with one of the keyboards we already have in the house, but they’re a bit big for the lounge, especially with the drums set up. The official keyboard is tiny in comparison, and you can hold it like a keytar, which means you can play it standing up (and look totally bad-ass, 80s-style, while doing so).
The MIDI output works really well — if you’re in the market for a small, cheap MIDI controller, it’s definitely worth considering, and if you’re in the market for a cheap keytar, it’s a no-brainer. The touch strip on the neck, which can be used like a whammy bar in the game, acts as a mod wheel in MIDI mode, and the Xbox controller buttons are all repurposed to cover useful features, like octave up/down controls and program changes; Create Digital Music has a great article that covers all of the MIDI features.
The one thing it lacks compared to a modern MIDI controller is a USB port, but when you can get a basic, class-compliant USB-MIDI adapter online for less than $10, it’s not a deal-breaker. I hooked it up to my laptop using one of those adapters, and within seconds I was pumping basslines out of XSynth-DSSI, using the touch strip to control the filter cutoff while I played. Awesome!