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.
tl;dr: Great-sounding, reasonably-priced microphone, purpose-built for the smartphones and tablets, with few downsides.
The iRig mic was introduced in 2011, and promised to be one of the best ways to get sound from your mouth or instrument into the iPhone 4, iPod Touch or iPad/iPad 2. Since then, it has been tested to work with newer iOS devices like the iPhone 5, and many Android phones. I've updated this review (in 2013) to reflect my more extensive testing since I originally reviewed the microphone in 2011.