I've been using MOTU on PC for about 3 years or so.
There originally were issues with the PCI-324 card which the rackmount unit communicates through. Once PC's started moving into the DDR memory and hyperthreading processors, there were SERIOUS issues getting the MOTU hardware to work with certain chipsets.
In fact, MOTU will still tell you that their PCI-324 card WILL NOT work with a motherboard that supports hyperthreading - though I can tell you that I currently DO run a system with the PCI-324 card on an Intel board that does support hyperthreading and DDR RAM. There were also issues with the 324 cards and on-board ATI video drivers, etc. It used to be quite a mess...and I built 4 different systems trying to get it to work properly. Once I found the right chipset (all Intel, by the way), it's all worked like a charm!
However...I just recently upgraded to the new PCI-424 card when I built a new system. This particular PCI card loaded drivers immediately and communicates with the PC/sound card flawlessly. Much improved over my last experience! Oh...and if you were wondering, new the PCI-424 WILL work with the older MOTU soundcards, so don't sweat it.
I'm running SONAR as well.
Maybe this was off topic...if so...sorry.