It's fluctuated between 2 and 4 dollars. Great, relatively unknown music app. With midi, you can use it as a controller; as an instrument, it's very flexible and has 8 voices (except on ipad 1) Get it! EDIT: Got a tweet, it's free to commemorate it's 1 year anniversary. It won't be free long. App info is currently processing and/or the app is not yet fully available for sale on the US App Store.
This is another good example of get wildly drunk, boot this up, tap wildly on your screen, and admire the heavenly experience. No wonder why Greyskull comes up with these apps!