tl;dr: For stereo audio input, mic input, and guitar input, there's nothing better. Sound quality is excellent, and the unit is physically solid—almost too solid!
[Update on iPhone 5 compatibility: Sonoma says the GuitarJack works like a champ with the iPhone 5 if you use one of Apple's 30-pin to lightning connectors.]
In my quest to find the killer solution for audio recording/input on my iOS devices, I've tested a ton of different external microphones and audio interfaces for the iPhone 4/4S, iPad, and iPod Touch. One class of device—a multi-channel input through the iPhone 4's dock connector—has been elusive until just recently, when three different devices were introduced a year after the iPhone 4: