True, an old PC (especially mine, a Pentium 4) uses more power, but I figure I can't be wasting that much electricity if I only have it powered on when I occasionally use it.
For a tech novice, I think building your own is a great way to turn TV watching into a productive habit where you get to learn a lot about computers. I find it really difficult to learn a new OS, programming language, or anything else if I don't have something I'm actually trying to get done with it.
If I were to do it all over again, I'd probably buy one of the new Android boxes and be done with it, though.