I've been using one since early this year. The only problem I've had is that I couldn't install Windows XP on it while the fan was hooked up to the temperature controlled "fan only" plugs. I actually just a few minutes ago plugged the fan into the "fan only" ports, and everything's running fine (I'd just been running the fan at full speed the whole time).
It's pretty quiet. Actually quieter than an X-Box I think, more like my Gamecube. The power supply is good, and it's really easy to work in. It's kind of annoying that the power switch is INSIDE the front door, so I basically have to have that haning open all the time. Goes great with a Seagate Barracuda V drive.
According to Intel's monitoring tools, it's running at 35° processor, 34° "system zone 1", 33° "system zone 2" with nothing but normal apps running.
Just ran through the Unreal 2 demo, and while it was running I got 50/38/40, which seems to be close to the safe limit (according to Intel's tools). The fan's sped up almost 300 RPM faster than before. (But what fan is that measuring? The case fan somehow? The CPU's fan?)
Now I'm back to the desktop, but running United Devices cancer research tool (which I'm normally running), and I've gone to: 49/38/40 and the fan speed's dropped by 50.
The rest of my system:
2.4Ghz Northwood P4 (533mhz bus)
Intel 845 chipset motherboard
Audigy 2
Geforce 4200 128MB (with RAM clocked to 4400 speeds by the manufacturer)
512MB PC2700 RAM