Reliability is a key factor for me with low-power being important, too. I've been running Mac Minis as servers since they first came out. Right now I have ~25 of them running at the MIT Museum, about half of those run exhibits and shut themselves off from 11PM to 8AM, a couple are used for desktops, the rest are running 24x7, some as servers, some as development machines. I've never had a Mac mini die. I'm in the process of moving our internal file server from a PowerMac G5 to a Mini.

We just got our first new, low-profile Mini and it's great. Not having to deal with the power brick is a huge win, and the power cord seats much more firmly than the old cords do.

I'm also experimenting with two Eee Box B202's running Ubuntu. One has been running 24x7 for the last 6 months without any problems.

Plug computers are interesting and for a while I thought they could make good controllers for touch-screen kiosks. But the iPad pretty much blows away anything we could put together with a plug, a USB display driver and a monitor (except in screen size).