Personally, I don't like apps such as this, as you're not really creating music but rather using already created samples and mashing them together without much customization. Another thing to note is that the dev, Ben Frost, has some pretty crappy apps out there; however, a lot of people seemed to enjoy the original dubstep version, one of his better apps. So again, I'm not really into this, but if you just want to mess around with music, you'll like it. Free until the end of this month. App info is currently processing and/or the app is not yet fully available for sale on the US App Store.
A better music app is free currently, might not be for much longer: Wavebot and Wavebot HD. Feel free to start a thread, I don't feel like retrieving the app-info, don't feel like being called lazy because I didn't post it, and already have it, so don't really care if anyone else misses out. At least, I don't care enough to catch grief for not posting the perfect post.