The hardware specifications of the TX100 S2 are as follows:
Under load, with all the disks spinning and the processor maxed out, the server uses about 50w. It idles between 20 and 30.
As soon as I took it out of the box and had the drives installed and configured, I installed arch linux (64bit.) So far, arch has given me no problems what so ever and the system is running very stable. The machine is running headless, so if I want to control it I either use FreeNX or an SSH terminal like TeraTerm. If I absolutely have to access it and I don't have an SSH or NX client available I installed shellinabox which is a ajax terminal client that runs very nicely inside most modern browsers.
The web server is running lighttpd in an effort to save a few resources that apache might other wise engulf. MySQL is installed, but not currently used for anything. I have plans to configure an IRCd on it, but at the moment there is little/no need for that.
Here's some current server stats, generated by webalizer. These are updated every 5 minutes. This page will update with them if kept open that long.
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Overall I have been incredibly pleased with the TX100 S2. The hardware reminds me, in many ways, of a cross between a thinkcenter and an optiplex or poweredge, which is a good thing as far as I'm concerned. It's both incredibly compact and easy to work in due to the fact that the drives are stored in a swing out bracket that is exposed when the case panel is removed. As soon as the bracket is swung out, full access to the logicboard is provided.
More information about the TX100 S2 can be found in this PDF by Fujitsu: ds-py-tx100-s2.pdf
A video of the TX100 being unboxed and setup can be seen here: