I use MythTV on a 2010 Mac Mini
I have a 2010 Mac Mini that I use. I installed GNU/Linux on it and run
MythTV. I can't speak intelligently on how well MythTV compares to Plex, XBMC, etc, but I've used Myth for many years and like it, especially for the automatic commercial skips, which works very well.
The 2010 model has an NVidia graphics chip so it plays HD great due to hardware access with
VDPAU. I had to set my profile down to VDPAU Slim to prevent the sound from skipping (it was exhausting the audio buffer too quickly when I had it at higher qualities). Once I did that sound was decent and video was great. I just have sound coming out of the television, though. I don't own a fancy surround sound system.
I'm not certain you'll get the same video quality from a 2011 mac mini, though, because I believe they replaced the Nvidia graphics with Intel. I don't believe VDPAU works with Intel graphics. Hopefully, I'm wrong.
You can run MythTV under Mac OS X if you don't want to dual boot with GNU/Linux. The quality was fair for HD when running Myth in Mac OS. Expect some stuttering when watching HD, but it's definitely watchable. If you want to try that, there are builds
here, but they don't have all the features compiled in, unfortunately. It's probably better to run the backend on a GNU/Linux box too, but the Mac mini makes an awesome frontend (at least for the 2010)