I own one.
Very, very small.
I bought the Celeron D version from CompUSA (identical to the Core 2 Duo version, just with less installed memory and a smaller HD).
I removed two screws, upgraded the memory with a standard DDR2 SODIMM (there are two slots). The system uses a standard 3.5" SATA hard drive (and not a laptop drive as you'd expect). It does use a notebook optical drive, despite what the specs may indicate. PSU is an external AC/DC style power brick.
I also upgraded the CPU. Since I knew it was Core 2 Duo compatible, I dropped in a E4300 with a pin-mod. Runs at 2.4GHz (1066FSB) without a problem, fully stable.
Regarding noise, there are two sources:
1) Hard drive - they used a Hitachi drive in my model that is obnoxiously loud. They might have used another brand in other models, I'm not sure. I'm not sure if or how I can enable acoustic management, or if it's already on. I would replace this with a quieter drive (since any SATA desktop drive works, try a WD SE16 or a Samsung T166).
2) Two 40mm fans for exhaust in the rear. Average noise levels during normal tasks; gets noisy when I load both cores. CPU temps at 2 x 100% load are around 55C. To fix this, I would lower the operating voltage on both fans. I'm just going to wire in a resistor for both fans to drop the voltage to 5V.
The size and form factor of the unit makes it impossible to install a bigger fan. But it is an incredible value: I paid $270 after rebate for the unit, $30 for a 512MB SODIMM and $150 for an E4300. Total came to $450, a lot better than the $700+ Newegg is charging.