I've been using MythTV pretty well along with XBMC. XBMC's add-on even lets you watch MythTV recording from a HD HomeRun Prime from XBMC and it all worked fine on Mavericks.
Now in the Yosemite world and also the latest XBMC now called Kodi and the new MythTV 0.27. Here are the steps:

1. Yosemite upgrade was pretty easy (surprisingly from Mavericks) and even seems a bit faster on my old and slow Mac Mini running it.
2. Kodi seems buggy compared with XBMC and now has audio sync problems more often. Fortunately, I'm getting pretty good at eyeballing the sync and it is on the play screen. It seems to be fine when using VLC, so it's a Kodi problem. Usually about 0.5-1 second audio behind the video. Also, it does seem to get confused about handling encoded HDMI input.
3. MythTV 0.27. This now uses MacPorts which is great. One tricky thing is that MacPorts requires a Xcode update and complete uninstall and reinstall on every operating system updates (?!!!). In my case, when I tried to do a sudo port update outdated I got a compiler does not exist error because it is trying to use the wrong compiler. So make sure you do a sudo port -f uninstall installed and it works much better