Someone left a nice review for Easy MIDI:
Great Idea and Well Executed ★★★★★
by Surface Tension
This app is truly great for producers on the go, especially when you don't want to drag around a huge MIDI keyboard with you everywhere.
-You can have two rows of keys to use for the piano, each with separate octave as well as "first white key" control so it can be set up however you like, and allows for hand arrangements other than C position.
-The "Chords" tab is probably the most interesting because it allows you to predefine chords and map them to numbers, so you can easily build a chord progression and play huge chords without being limited by the number of keys that you can press simultaneously on the keyboard.
-For example, "1" could be mapped to a chord with up to seven notes in various octaves, and "2" could be mapped to more notes, then playing both simultaneously allows you to play enormous arrangements that wouldn't even be possible with two hands.
-Allows you to use mouse and touchpad input and gestures to control the mod wheel, pitch bend and note velocity
-Haven't used it enough to discover very many, but there is no way to make it constantly override keyboard input, so if you were playing notes in an application that doesn't support keyboard MIDI input (Pro Tools!!) and you wanted to tweak your synthesizer's sound, you would have to switch applications back to Easy MIDI in order to test your changes.
-In any case, this is something you would probably use a MIDI controller for, so it's not that big of a deal
All in all, this is a great tool for not just playing notes on the fly, but also for easily mapping and saving chord progressions, so you dont' have to export MIDI files between programs.