Worth it for crowded hallways, not so much for voice notes.
My smartphone is already how I do my banking, navigation, most of my communication and nearly all of my flashlight-ing. But, when I have to conduct interviews or record the drunken ramblings of friends for posterity, I usually turn to my camera and directional mic. Then I heard about the i-Microphone, which plugs right into your headphone jack to provide improved audio pick-up.
GoBiz was good enough to send me a sample unit, which I set to work with right away. I hassled all of my friends and neighbors to help me test the audio at five, ten, fifteen feet and twenty feet. I used the Voice Recorder app to conduct all tests.
The Bad: Your phone can't output any audio when this thing is in. You'll need to remove it to place any phone call or even to hear your phone ring. I don't think there's any way for GoBiz to get around this, but it's something to be aware of as a user.
General Performance: At every distance interval, the i-Microphone sounded notably clearer and louder than the Evo's stock mic. At five and ten feet, both microphones had fairly clear pick-up. At ten, the Evo started to fade, and by fifteen I could no longer distinguish most individual words when outdoors.
The i-Microphone was clear as a bell at up to 10 feet, and clear enough to discern without trouble at fifteen feet. It was less than ideal at 20 feet, but still clear enough that I could make out what was being said.
Conclusions: If you want to use this thing to take notes in a crowded lecture hall or classroom, I'd suggest sitting in either the center-front, or top rear. Place your phone with the i-Mic facing the instructor and just let it sit. Indoors, I expect you'd be able to get a decent sound even at twenty, twenty-five feet.
The i-Microphone MSRPs for $25.99. It works well, but the difference between it and the stock Evo mic is not meteoric or anything. For close-range interviews and personal notes, the i-Mic isn't really necessary. If you're looking to record outside or in noisy, chaotic environs though, this is a cheap and simple way to get the sound bites you need.