I bought the Lemur Ipad app last year so i could control the remix decks in traktor. Not really used it much to be honest, but its a veritable powerhouse of a modular design it yourself midi and osc controller, and a tiny amount of what the original costs.
Theres a load of user mappings you can download, and it comes with this very nice ableton controller - Live Control 2 - for free. Here's a video