at this point in time, the Microtrack cannot record a 24 bit S/PDIF data stream. that's the biggest one for me.
that and just general instability. The unit shouldn't freeze up for no reason.
and Frank, thanks for being in touch with M-Audio, hopefully they'll get some new firmware ready soon.
Edit to go into more detail:
last night, I had the Microtrack setup to record at 16/48, via the S/PDIF input. I had access to AC power, and the microtrack was plugged in. pre-show, everything was fine. then the band came on stage, I hit record, and the unit froze. after holding the power button for 20 or 30 seconds, it turned off. I then turned it back on, hit record, and all went well. I ended up starting the Microtrack approx. 2 minutes after my DAT. at this point, I don't think it's stable enough to completely replace my D8.