I have tried GBPVR, Freevo, MythTV XPMCE.. and the best of the bunch so far is Vista... although XBMC is pretty funky too!
No DVB-S support without third-party hacks, and then no HD (for Europe) support is a dealbreaker for me. Microsoft don't seem remotely interested in fixing it.
No ability at all to cope with both DVB-S and DVB-T tuners simultaneously - you can choose one or the other, and that's it. Don't even think about trying to use more than two tuners of any kind, either.
It refuses to FF/rewind through avi, mkv files etc - there's a third-party plugin that will provide some of the functionality that's (inexcusably) missing, but it doesn't work very well in my experience.
Don't for the love of god spend ages downloading high-quality album art, name it according to the "folder.jpg" convention, and then let WMP/VMC loose on your music collection... before you can say "oh, bugger," it will have overwritten your image files with nasty low-res 200x200px replacements without so much as a by-your-leave. Yes, you can configure it not to do this, but by the time many people find out it's too late.
Oh, and just give me five minutes alone with the twat who came up with that "wall of thumbnails" idea in the video library... a more useless way of organising media is hard to imagine. Not only is it impossible to identify a particular file in the ugly, confusing, cluttered mess until the cursor is actually on it, but you can't even easily go through them in a logical order because of the stupid up-down-across way they're arranged.
I could go on, but my blood pressure is getting dangerously high already...
Of the Windows alternatives I've tried, I think SageTV seems about the best, although the latest version of GBPVR (1.2.9) is rather nice, and miles better than it used to be (and free of course, unlike SageTV). As far as MythTV is concerned, never was the saying "Life's too short for Linux" more appropriate...