The price is US$4.99, which is actually a little high for a satellite application -- you must be running djay 3 on your Mac, and while there is a free trial, it would have been nice if they'd just released the iPhone app as a free add-on to the $50 app. But as you can see, it's a full controller for the app -- they have the vinyl artwork included, you can set cue points and tweak FX and audio levels from right within the app. Also, you can queue songs up for an Automix directly from the iPhone (if, say, you happen to be at a party and don't have time to run upstairs to your computer to add a little more music to the playlist).
The app works over WiFi, and note again that you must have the djay 3 software running on your Mac -- this is basically a remote for that app. But if you do a lot of DJing from your computer and want to get even more remote, setting this up on your iPhone seems like a great solution.