I bought the Korg app few days ago, 10 bucks, is good, little complicated, still learning..

I also bought Isequence today, is very good..around 15 bucks..

And my favorite is LooptasticHD, great for making tunes and beats.. Around 20 bucks..

I think all 3 are great, and recommend if your into making music.. Nothing to be your main source, but great for experimenting and creating rhythms and beats..

I have all three and LooptasticHD is my favorite too!

The Korg app is awesome.

DJ apps all share the drawback that there are only two output channels (left and right), so if you use one to monitor in the headphones, the main out is only mono. I think controlling a DJ on your mac with wifi has more promise. (ahem, traktor, get on it)

Amplitube is awesome, too.