I built two on Biostar 200N chassis, but in the end it's very hard to beat an upgraded Sky+ box. Is there anything you specifically want to know?
The big plus on the Epia is the silence with 'low' cost, but if you use Socket A as a platform you can get the same cost and much more performance. Socket A can also be had now with effectively passive cooling so again that's not a big issue.
Most people are now looking at Pentium M as the processor of choice for HTPC, but they are not as cheap as the Epia solution.
As I said - in the UK you can get a Sky+ box for very little (£150 without any contracts) and it's a real dedicated HTPC.