Just a few months ago your only option for a customizable iPad MIDI controller was TouchOSC. New competition is coming at them from every side, and seriously raising the bar. Beatsurfing is the latest with a unique aesthetic, that includes overlapping controls, and some original features like having your controllers control each other for chained events.
Like TB MIDI Stuff and MIDI Designer, you can make and edit your layouts right on the iPad. As an added bonus, this fully supports Virtual MIDI for controlling other apps!
Here's a nice overview from the developer, DRUW.