Review: iRig Mic Cast
tl;dr: Offers little more than the built-in iPhone microphone, but it's a good mic for the price, and is very useful in certain situations.
Since adding my comprehensive overview of audio input and microphone options for iPhones a couple years ago, there have been many purpose-built microphones that are made particularly for smartphones. IK Multimedia's iRig Mic Cast microphone is one of these purpose-built devices.