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.
The iRig Mic Cast is a quarter-sized microphone that plugs into the headset jack on any iPhone, iPod Touch, iPad, or Android smartphone or tablet (as long as it has a standard headset jack), and allows you to record crisp, clean audio.
It has a Lo/Hi level switch that attenuates (lowers) the sound level for different recording situations, and includes a handy plastic stand so you can set your iPhone on the desk in front of you while recording.
I recorded a quick video review (below), and after that, I'll get into a more detailed review.
Video Review
Build Quality
The iRig Mic Cast is built with a tough plastic shell and a thin metal membrane over the mic capsule itself. The TRRS plug that plugs into your iPhone's headset jack is molded directly into the Cast's case.
I don't think this little mic capsule will be able to take a beating like a trusty metal Shure SM58, but it feels solid, and has survived a few drops already. The switch might be the first thing that goes out on this mic, since it's a very small bit of plastic that requires some fingernail pressure to move—but I imagine most people will leave it switched to the 'Lo' or 'Hi' setting forever, so that's a non-issue.
The design of the Cast means it will stick out of your iPhone's top (iPhone 3GS/4/4S) or bottom (iPhone 5). Luckily, most recording apps will work in upside-down orientation, so flipping your iPhone 5 to have the Cast on top won't be an issue.
Audio Quality
In progress. Please check back soon!
Recommendation
The iRig Mic Cast is a hard product to recommend. For the iPhone 4/4S, since it is on the 'top' of the iPhone, it allows you to have the iPhone in a vertical orientation on a desk in front of you and all recording apps will work normally. The mic faces you better than the built-in mic, and gets slightly better pickup than the built-in mic. On the iPhone 5, since the iphone is upside-down, not all recording apps support the orientation well, and the iRig Cast is right where the built-in mic is, offering little better pickup.
However, the iRig Mic Cast does offer a built-in limiting switch, which will help greatly if you're recording a loud sound source (a guitar, an amp, a piano, etc.), and produces slightly more directional pickup than the built-in mic. So there's that.
I'd recommend the iRig Mic Cast if you'd like a compact little mic for recording podcasts or live music sessions with your iPhone on a table or desk in front of you, or maybe for simple reportage recording, but there are many other mics and audio input solutions out there that offer higher quality and more flexibility. Some people may find the iRig Mic Cast to be a valuable addition to their microphone arsenal, but I don't.
You can read more about the iRig Mic Cast on IK Multimedia's iRig Mic Cast product page.
Buy the iRig Mic Cast from Amazon for $32.