I used a Blue Mikey Digital with my older iPad. Really impressive sound (TwistedWave audio app - great for 2 channel recording - a bit like SoundForge for IOS)
The mic is really good and has the bonus feature of including a line in connector as well. Like ljguitar and others I got pretty frustrated with the connector change to lightning, so I have gone the external route and use my better studio mic's. The built in mic is good enough for "on the go sketchbook" stuff.
They made a newer model with lightning connector, but I didn't want to drop another C-note to get the same mic just for a connector change (realize there are adapters but it gets kludgy and more stuff to lose).
https://www.bluedesigns.com/products/mikey-digital/ - Note: only available used now