This is a gorgeous app that represents keys, chords and scales in a unique layout. It comes with 4 Sounds, and they'll sell you additional sample packs, but the real treat here is that it can be used to control anything with MIDI. Including CoreMIDI apps that support background audio, such as Sunrizer or NLog Synth Pro! This is a fun and inventive way of playing that is inviting to new users unfamiliar with music theory, but also useful if you do. For more experienced users you may also want to look at PolyChord. There is an optional pseudo-velocity for your screen presses, with adjustable curve. I found myself wishing there were some controls for MIDI CC, or at least a mod wheel, to interact more deeply with the MIDI slave. You have the ability to use your device's accelerometer for assignable MIDI CC, but this is a little impractical on even the light-weight iPad 2. A free version of SoundPrism is available here with pretty much all of the features, but none of the MIDI. The documentation and tutorial videos Audanika has put out in support of their app is commendable!