When I bought this mobile interface in 2002 it was exactly what I needed and fullfilled its purpose with no problems. I was using Cubase 32/VST in an iMac 600mhz. The sound quality was smashing, crystal clear, although the two mic-preamps had a slight bit to much noice to be called "proffessional" mic preamps but I didn't notice it at the time.
Moving up to OSX the problems appeared. Way to much input noice due to bad drivers. In order to reduce input noise I must switch on monitoring in my recording software (to hear what's happening) and change the settings in the Duo's control panel. Not that it sounds better with a certain setting it's just that I have to fiddle around with it till it suddenly becomes quieter. So for instance,using max latency and lowest bit-rate didn't reduce the input noice, neither did the opposite settings, I just have to fiddle with it back and forth and all of a sudden the input noice will reduce. Very weared indeed.
The Duo has individual pad switches for the XLR-channels + one line level switch for the to TRS witch is rather nifty.
S/PDIF output on RCA but no input.
Stand alone mode if you want to use it only as a mic-preamp.
Altogether a good hobbyist A/D Converter.
I wouldn't give m-audio much credit for compatibility and driver stability though.