I used JRiver Media Jukebox for years and bought multiple versions of Media Center, which i also used for years -- and I loved it and still loved it. But I haven't bought the latest upgrades, because:
When I switched to a Mac for my main computer, I bought Parallels and installed Windows XP on my iMac almost entirely because I wanted continued access to Media Center . . . An investment of time and money just so I could have the JRiver goodness.
But slowly I stopped using MC, because as I got more into using the Mac OSX and programs, I didn't want to bother running Parallels most of the time anymore -- because it really was just MC that I needed in Windows. And now I've switched over to all Macs -- iMac, iPhone, Macbook, etc. -- and I don't miss Windows at all. And I've had to grow used to a mishmash of media players -- iTunes, VLC, etc . . . But I miss JRiver Media Center. If a native Mac version of MC ever comes out, or a solid, JRiver-official Wine port or a cross-platform java or webkit or whatever version that still has most of the MC bells and whistles -- I'll be first in line to buy the JRiver Apple MC.
(I still use Media Center on my work PC -- a crappy old Dell -- but since I maintain my main media library at home -- 30,000+ songs, hundreds of movies and TV shows -- all I need at work is basic music playback of my at-work playlist, which MC is great at, but within this context it's no longer a must-buy-the-upgrade program for me. I know it's a big ask, but please get us a Mac MC some day!)