TonePad 4+

By far the most enjoyable music app yet. Some of the other synths on the iPhone are impressive through features and complexity, but this is so simple, well put together and astonishingly enjoyable to use that it beats them all. And it's free too! I would pay for it if they could give you a few different tones to use, and maybe even a second matrix for a filter, switchable between bandpass and lowpass...that would make it Awesome! Install it now!

Pros: fun, easy interface, made for touch screens, instantly musical.

Cons:
resulting ringtones cannot easily be used as ringtones on iOS - no really: the whole USP is basically a red herring - major design gap here;
the resulting ringtones lose the echo that gives them that musical charm in the first place: another BIG one;
There is no way to change the modality of the composition!! Happy ringtones are great but I expected a bit more bang for my buck - this isn't rocket surgery;
There is no way to change the length of the composition or concatenation existing 'squares' - again you'd think this wouldn't be too much to ask for a paid iOS app but apparently it is.

Looking at all this I don't even think it's worth 3*

I thoroughly recommend this app - it's strangely fun to use and is quite easy to play with at length. As at least one reviewer has mentioned, it can be hard to hit the right 'dot' but since I have the same problem with the iPhone keyboard, i think it's an iPhone thing and is no refection on the app. It'd be nice to be able to have a longer loop by perhaps joining two or more 'songs' together. Altogether though, this app is great fun and best of all is free.