The Tascam iM2 is a very nice mic for the money. It uses the 30pin port, and not the mini-stereo jack. I have used mine on an iPhone4S along with the free Tascam recording app, PCM Recorder, and have gotten some pretty excellent quality recordings.
In general, I would favor devices that connect through the 30pin port over ones that use the mini-stereo jack. Much better recording quality. Of course, that has been put in question by the intro of the lightning port for the iPhone5, but I think all of the iPads still have the 30pin port.
LIFE is good - remind yourself of that every day
MIA Deluxe Jazz
Turner Ren 4 fretless