The app is free to download and includes all functionality and 1 free instrument. You can get additional instruments by liking us on Facebook and tweeting, or through in-app purchases
Looks like people want more than 1 instrument to create a song with.
The restriction isn't letting people get a feel for the potential of your app.
I would like to recommend it to my fellow electronic musicians, but with such restrictions it is literally not worth the time. Make a lite version that presents the interface and 2 or 3 instruments simultaneously, and a premium one that just unlocks everything. There's a huge variety of electronic music software on the iPad, and musicians don't have the patience to be nickel-and-dimed.
In my experience with iOS music apps, most of them use IAP. YMMV.
I guess I'm biased towards the more pro end, though some pro SW like Animoog does that too.