From everything I have read, you don't want to build your own HTPC unless you do enjoy playing around with the wires and software as well. It turns into a hobby all its own, with some of the tweaking that goes on.

I'm holding out for the ones with cablecard built in, because I really want it to record HD more then anything. It looks like those can still only be purchased OEM. While I would probably prefer to build myself, I'd consider just buying a HTPC if the price was right.