This is the latest addition to my DJ battle chest. The Midi Fighter Pro:
Because it has no default functionality, when you get one you need to find a midi mapping file for it. The midi mapping, expressed as a .tsi file for Traktor Pro, interprets button presses as program commands and allows you to assign software functionality to hardware controls. Traktor allows a user to specify actions based on certain conditions (if a song is playing, if a numerical modifier meets a certain condition, etc), and this gives way to some really complex, expressive functionality. I've recently finished making my own mapping for it, which revolves around being able to quickly access a huge variety of effects, and control them precisely. There's a video below, and here's a link to a page where you can download the mapping.