Musical instruments are extremely expensive these days. It could cost a couple of hundred dollars just for a half decent guitar and even more for a drum set. Carvware has helped you out a little bit by developing a program, with the help of Tattiebogle's controller driver, called Gamepad Companion. This allows you to play your Rock Band instruments through garageband via the USB input on your Mac.
The process of setting up your drums is pretty easy to do once these programs are downloaded, both of which are available for free. It works by having the programs calibrate your 360 controller or Playstation 3 controller to your keyboard. You then use the musical typing feature in Garageband, and you're pretty much set.
This software is only really beneficial if you're using the drum set. The guitar does not give you much diversity in terms of notes to play, but it can still be used, though don't expect anything exciting.
The drums are not perfect either. There is a slight delay that keeps you from really jamming out. It also has issues if you're playing very fast, as it tends to miss notes or not pick them up at all. For a skilled drummer who plans on going crazy on this, then it's definitely something to pass over. For those of us who want just to do basic beats and drumming though, it would work great.
This is a cool little trick to do with your drum set. It can be done with both the Playstation 3 and Xbox 360 versions of Rock Band. In the end though it's going to leave a lot to be desired with the delay. Try it out and see what you think, but don't expect to get a $300 drum set out of your Rock Band drums.