Great topic.
I'm currently using USB Aux/Send/Return with my iPad, specifically the Launchpad app. I fire off one-shots primarily, but there are a few loops as well in some of my kits. The Catch 22 is that Launchpad isn't capable of MIDI In, so syncing the BPM via MIDI is out of the question. That said, I find the tap-to-tempo function in Launchpad works fine since, as previously mentioned, I'm using primarily one-shots/FX (ex: sweeps, uppers/downers, bass drops, etc.).
My hope is that the brains at Pioneer will develop a system similar to Ableton Link that will enable App developers to implement a protocol that will enable their apps to easily sync within the Rekordbox stratosphere of an NXS2 setup, similar to how the RMX-1000 app magically syncs currently. Such functionality would truly put the mixer over the top and introduce unparalleled creative possibilities.