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..