i'm no audophile but from what i've read part of it has to do with issues around a lack of shielding that introduces background interference noise from the fans, power supply etc (in fact the criticisms of the iMic for some were that the usb cord is too short to get the analog signal far enough away from interference before encoding)
- that said, most of the stuff i've read about audio recording on macs suggested that the iMic was better than using the mic jack, but that it was still a big step below having a dedicated soundcard input or something like the MBox (but then it's a hell of a lot cheaper).
I did a reasonable amount of research on it, because i wanted to rip my vinyl collection and the mic jack just wasn't giving me the signal i wanted (as mentioned above), and ended up with the iMic as the best compromise between price and quality.
and i'm converting them all to mp3s so the finer details of the recordings that i might have picked up with a fancier system would probably be lost anyway (had i been able to afford it).