I use my iPhone and iPad all the time. The iPad makes a SICK MIDI controller (it can do most of what the Lemur can do, if not more).
Here's what I said in another thread
So far I've just been using TouchOSC and been making custom layouts (I'll upload a video of my layouts tomorrow or so, as well as the PureData and layout files if anyone else has an iPad). TouchOSC has got stuff like buttons, toggles, knobs, faders, fader arrays, X/Y pads, and tons of other stuff. It's somewhat of a bitch to set up, because, at least on Windows, you've got to create a PureData patch that takes the OpenSoundControl data and maps it to MIDI data (one of many ways to do it), but the fact that it's a fully customizable MIDI controller is fucking awesome. You can also use it with Max/MSP but I've only just begun coding that. And on Macs, there's an application called Osculator that makes the entire process of OSC data to MIDI extremely easy. Basically, it can do pretty much everything the Lemur can, plus iPad apps, web browsing, music, videos, etc, so IMO it's far superior.
Anyone who's interested should go here and download the TouchOSC editor to see how cool it is to make a layout. You can put any kind of control you want anywhere, in any color, any size, any proportion.
http://hexler.net/software/touchosc
It's almost completely replaced my MPD, because unlike my MPD24, which has 16 pads, 6 faders, and 8 knobs, my iPad can basically have an unlimited amount. It's even got pages where you can switch to another layout with the tap of a button. The only downside really is not being able to physically touch the knob, fader, or pad, and the fact that its not velocity sensitive.
On my iPhone I use BeatMaker on a regular basis, with loop packs exported from my PC.
Recently I stumbled upon an app called NanoLoop
http://www.nanoloop.com/iphone
It's a port of some GameBoy homebrew, with a few more fancy features added. I've actually been really surprised by the sounds I've been able to get out of it. It does some basic synthesis, and it can load .wavs from your iPhone, which you can add via email (or through FTP if you jailbreak). EVEN MAKE PHAT WUBBLEZ http://phiik.com/bass.wav
There's also SunVox http://www.warmplace.ru/soft/sunvox/
It's a port of a multi platform sequencer. This thing is far more powerful than the last app, but a bit more complicated to use, mostly because of the fact that the entire app is squished into the resolution of an iPhone. It's also tracker based, which I fucked hate. It does some sweet stuff though, even has effects and all that jazz.
Really powerful, for example, this song was made with SunVox
ftp://ftp.scene.org/pub/music/artists/observer/2009/mp3/nightradio-emergency_exit.mp3
will probably use it as a live instrument or record some bits and pieces in for my tracks eventually
