The driver seems to be unstable under heavy CPU load. I am using this Transit USB on a 400 MHz Ti-Book, with fairly clean 10.3.x through 10.4.5 installs. Audio problems are exacerbated by heavy CPU load, but can also happen under a light (< 20% CPU load). At random times it will begin a 1 second loop over and over, adding any sound played to it to the loop. This may happen to record as well, I cannot recall. Usually going into the M-Audio preference pane and changing a setting will fix it, but this time my machine locked up requiring a hard reset.
I previously used a Griffin iMic with no problems, and wish I had sprung for an Edirol for my optical interface.