A PC in the living room just amplifies why PC's suck... they are bulky, noisy, crude, and require way too much setup, maintenance, monkey work with drivers, codecs, etc. They are the antithesis of what a HTPC should be.
Now, you can build fairly quiet, compact PC's with Windows 7 media center that will rival a Mini/Plex experience, but you won't save much (anything?), and will still be battling with drivers, codecs, etc.
I had a Vista HTPC that was no end of trouble. Every NVIDIA driver update would create new problems.
A HTPC should just work... and that's a Mini+Plex.
Also, there are benefits to a complete Mac ecosystem.
On the other hand, a Windows based HTPC may offer advantages in terms of integration with TV tuners (something that doesn't matter to me).
The only disadvantage for me, and it's minor, is that the Mini doesn't do video+audio over HDMI and therefore no 7.1 channel sound and there's also two cables instead of one. I'm keen to see what Monoprice can do for us in the dept. with their display port + USB sound to HDMI cable.