The volume knob on the front of the FF400 would sometimes stop working. I could still change things in teh software with a mouse, or with keyboard shortcuts. It was this randon thing that happened all the time, was very frustrating when you need to check music at different volumes for a living
Also, the FF400 was very picky about the order you plugged it in. I basically always had to reboot the laptop every time I plugged in the FF400 or it would freeze the system when an app tried to use it.
Both issues appear to be fixed in the new 2.72 drivers.