Its USB 1.0, bit of a pain as USB2.0 would allow you to run an external drive for storage - you can do it with USB 1.0 but its slow as anything (12Mbps, where as USB 2.0 is 480Mbps).
There are no PCI slots so you can't plonk in a USB 2.0 card either...upgradability on them is strictly limited so make sure it'll do what you want it to from the off. No AGP either, onboard graphics. Mine requires both USB keyboard and mouse as well - no PS2 (some are legacy free, some aren't!).
Oh, to disassemble them you need a star bit key! Just thought I'd mention it, save you end up buying one and the necessary extra bits (RAM, HDD etc) and then (like me) finding you can't start to build/setup the thing cos you don't have the tools to get it open
I'm unaware how similar systems compare, I think there are plenty of others out there though - I know Dell do an even smaller and much nicer looking SFF PC, which is P4 based, as the library at uni has them. Much more oomph hardware wise, but cost more as they're newer and possibly a tiny bit noisier.