Aurora Sound Studio HD. I bought the tool when it reduced its price from $39 to $14 though it is still on the high side.
Aurora is very similar to the TNR-I (Tenori-On) app, but it tries to be a bit more of a full DAW with exporting, sequencing etc rather than a performance instrument like the TNR-I. I say "tries" because once I had painstakingly sequenced in my patterns, I exported the file to WAV format and was disappointed to see that Aurora distorted two of my patterns where a smooth sweep became a staccato mess. This prevented me from putting up the piece for so long that I finally gave up, put the headphone jack into the mic-in and recorded it via audio.
The audio output of the headphone jack on the iPad is quite noisy so I used a noise-gate in Logic-9 to clean up the resulting audio and I think it came out ok. I may use this technique for other synths that make recording difficult (such as the TNR-I and the excellent but hard to record AniMoog).
Using the 16x16 grids for sequencing is a novel approach and in some instances I like it better than banging on fake piano keyboards but it can be very time consuming to get a full piece put together. I think as far as push button sequencing goes, I like the TNR-I a bit better but Aurora does provide more functionality - if only it would work properly!