No brainer indeed they're great. I'm on my third one. However, they can trip you up if you're not careful. Also why not build on? It's no harder than lego & you get A LOT more bang for your buck.
Here's some pics - sorry for the ugly Frenchman in some of these, but Sky Sports news is a good arbiter of picture quality. These are freeview broadcasts via a Hauppauge Pvr500 & an xfx 8500gt which is really overkill for telly.
and here's the machine:
You will get a slightly blocky picture on bright colours & fast motion, but it's nowhere near as bad as you may think. The obvious pitfalls are do you really want a pc in the living room - this is often cited as the main resistance to the wide uptake of media centre, but as you can see, pc's don't have to look like pc's. However, unless you go big time upscale on cooling, it will hum away in the corner.
Don't go sff, they're limiting in every way. If you do build & why wouldn't you? then take advice from the communities of geeks (I.e. me) who love these things. Green button, av forums, google around.
Enjoy.