I had no problems at all with this interface in Windows XP. Sonar just profiled the device and away we go. I initially thought I would never use the SPDIF inputs but then I got the Roland Fantom, which has digital audio out. Apart from being lower in volume, I figure it has got to be better, right? So I feed the Fantom digital out into the SPDIF on the Delta-66 card.
Since moving to a Windows 7 64-bit workstation, I have had some issues. Occasionally I'll hit the playback or record in SONAR, and instead of getting sound I'll get silence. I've learned to hit the stop button pretty quickly when this happens, because if I don't I'll get a pulsing pile of digital rhythmic feedback noise on my electrodes.
I suspect the Win64 drivers from M-Audio are to blame. It happens whether I select ASIO or WDM/Kernel streaming mode, and is not affected by buffer sizing.
If M-Audio don't produce updated drivers that solve the proble, then at some point, I am going to have to switch to a different vendor. Probably Echo Layla 3G.