Mad Catz is the official hardware partner for Rock Band 3, and their two new realistic instruments—pro guitar with 102 buttons, and the keyboard peripheral—are the closest you get to real instruments yet.
The $150 Rock Band 3 Wireless Mustang Pro Guitar Controller works pretty much like an analog to a real guitar, in that instead of pressing down on the frets in certain positions and strumming, you hold down the corresponding fret button and strum. You can move your fingers up and down and wherever—just like on a real guitar—and the game will recognize all this in pro mode.
It's also MIDI compatible, so you can plug this in MIDI style to your Garage Band and actually make a real song. Or play out the song you just learned how to play in Rock Band 3.
The guitar is dissassemblable, and the nylon strings can be replaced without replacing the entire guitar, if you somehow rock too hard and break one. Try not to rock too hard.
The keys themselves feel like keys, and are neither too rigid nor too mushy. They're the halfway point between using the actual Fender real guitar (with strings and everything) and the traditional 5-key plastic guitars.
What's also great is that you can use the outermost 5 rows of keys as a traditional Rock Band controller, so they map to the colored buttons in a standard controller. That way someone who isn't up to the challenge of hitting 102 keys.
The $80 Rock Band 3 Wireless Keyboard Controller, which is available as a bundle with the Rock Band 3 game for $130 if you already have old guitars and want to just use them. It's 22 keys, two octaves, and actually feels like a MIDI keyboard. As a dude who had almost a decade of miserable piano training when he was a kid, it feels good. Like a MIDI keyboard should. And you can even plug this in MIDI style to your Garageband too.
Easy keys mode is basically the equivalent of easy mode with the Pro guitar. Five of the keys map to the colored buttons coming down the screen. But in actual keyboard mode, buttons on screen map 1:1 to the buttons on the keyboard for a more enjoyable/realistic/learning-how-to-play-a-keyboard experience.
As a side note, Harmonix and Mad Catz don't like it when people call this a Keytar. But keytars are COOL, and this thing has a neck and a strap to be played in guitar mode. So it's a keytar, and it's a keyboard. There's nothing wrong with keytars. Be proud.
(If you're a good keyboardist, you can also use the keyboard as a bass or a guitar in previous Rock Bands as well, instead of having to swap out and strap on a guitar for every other song.)
Then there's this $40 Rock Band 3 MIDI Pro Adapter, which takes in MIDI keyboards, MIDI guitars and MIDI drums so you can use your own instruments with the game. You need one adapter per instrument, and you can toggle between instruments directly on the adapter. And it has a belt strap.
The $40 Pro Cymbals Expansion kit is also an improvement on the previous cymbals add-on, being permanently angled at 10-degrees instead of every-which-way.
With these pro instruments and adapters, Mad Catz really enables Rock Band 3 to feel like you're learning an instrument rather than pressing buttons that approximate instruments. It's true that to get all these adapters and guitars and keyboards will cost you a couple hundred bucks, but it's the first time you can justify the price with the fact that you can use these instruments as actual MIDI instruments outside of the game.