Been playing with the new Traktor iPad app over the last few days and it's good. I've previously been using Traktor Pro on my laptop with vinyl controls.
There's plenty of DJ apps out there, most of which try to mirror the two deck system and put turntables into an app. You can clearly see that Native have set out to create a true iPad DJ experience and playing to the strength of the tablet with multi touch controls.
You have two channels, stacked on top of each other, cross fader, FX, EQ, syncing, BPM count and key on all tracks in your library.
The library has a nice touch and swipe interface, and you don't loose sight of the music while your rummaging either. on the right hand side you have suggestions for tunes that are the same BPM and key so should mix nicest.
You can keep it one channel out or use the split and team it up with a dual out cable for master output and cue in the headphones. It also records in app, it syncs with your drop box so you can pic the mix up on your phone or desktop automatically. It also has master and cue volume controls.
Below is the connector I'm using to have a master out through speakers and cue in the headphones for the full experience.
Just recorded the first mix with it, you can hear it here. The only downside I have noticed when recording or using the master out is depending on what you're going through you can experience delays in play back. So straight into the stereo/amp is best.
DJing is becoming easier and easier as well as cheaper. I'm always a little cynical about digital DJing and how it seems a little too easy compared to vinyl but hopefully more and more good stuff comes of it. I think NI Traktor leading the way in iPads for now...