IK Multimedia iRig MIDI by Whitecat
The iRig MIDI is a super-fun little tool that, when paired with an iPhone, does indeed turn your iPhone into a mini-workstation.
I'll review the IK software options later on, but just speaking for this as a MIDI device, it does exactly what it's supposed to, which is transmit MIDI data to and from the iPhone. The Sampletank app obviously works well with it but there are other apps that can sense MIDI input as well.
It's very lightweight - one critique might be the plasticky feel of it is almost too flimsy, but of course part of the whole point is ridiculous portability so there's something to be said for keeping it light.
Two LED 'arrow'-shaped lights above the in & out labels activate as data passes so you can see what's going on. Useful for connection diagnosis on a dimly lit stage!
As far as connectors go, it uses 2.5" TRS mini jacks for the MIDI I/O (in/thru/out) - the adapter leads (included) are jack-to-MIDI male connectors. The kit also includes a USB power connector so you can keep your phone "on charge" during a show, for example.
To get it going you do have to download an app from the App Store called iRig MIDI Recorder, which will then provide a few options for MIDI data handling (such as OMNI listen on or off), a method for future firmware updates (if needed, mine's still 1.0.0 I think!) and it can also capture MIDI recordings which can then be exported.
Beyond that you simply pair it with the MIDI-aware app of your choice. I have mainly used it with SampleTank for iOS thus far (with a Korg K61 controller) and had no issue whatsoever.
So, not much else to say as this isn't a device that makes any noise on its own. If I had any further criticisms, there's probably an argument for having a single-connector 'breakout' cable rather than four different ports/leads (think Apogee Duet) - but of course that might not be to everyone's tastes as if you lose the one snake, you lose it all! That said, replacement individual MIDI cables from IK are 8 euros each plus VAT and delivery so don't make a habit of misplacing those either.
Anyway, anyone with an iPhone and a spare MIDI controller should really check this out as it will fit in your pocket and you can make some surprisingly good-sounding stuff with the range of apps out there.