The DJX is a Yamaha Synthesizer aimed at DJs.
It features:

• 128 XG sounds version of the standard GM sound set
• 159 dance-oriented sounds, including an excellent B-900 Drum kit.
• A few effects
• many knobs (volume, balance, groove, resonance, cutoff)
• A ribbon controller that manages tempo changes by default but which can be assignated to any other MIDI controller
• Loads of dance oriented styles
• A 64kb sampler with compression mode, allowing the capture of up to 6sec phrases.
• A dynamic keyboard sensible to aftertouch and velocity
• decent stereo speakers with bass boost
• An arpeggiator
• A midi sequencer

In pattern playing mode, you can activate or mute any pattern track as well as twiddle it with the controller (resonance, cutoff, etc.)
Though it was aimed at being synchronized with records in order to remix these, it lacks an automatic tempo detector that appeared in the newer DJX2 (which lacks the arpeggiator and the dynamic keyboard).
It is quite cheap ($200 boxed) and its sequencer lacks ergonomy but it will reveal its power if controlled from an external sequencer.
For more info, please visit http://www.yamahadjx.com