I have the Media Center upgrade, but it's not really necessary if you use MC18. Media Center is basically exactly the same as Media Center on W7, you might install it if you wanted to play DVDs, which W8 isn't able to do out the box, but MC18 handles that pretty well, I think. I don't think it does anything to improve sound quality, or what not. Someone correct me if I'm wrong.
You can play DVDs just fine on Windows 8 inside
JRiver MediaCenter. I don't think you have DVD playback support inside
Windows Media Player, unless you have the
Windows Media Center add-on installed though.
Personally I prefer Windows 8 to Windows 7 by quite a lot. There are a lot of little new features that all add up. (explorer is a lot better, task manager is much improved etc.) But I had to buy
Start8 to bypass the Metro start screen, as I don't like it at all.
But if all you're running on the system is MC18, then it probably doesn't matter whether you are using Windows 7 or 8 I suppose.
The only outstanding issue I have with Windows 8 is that, with Nvidia at least, when the graphics card is connected to a TV, it only outputs 59.94Hz or 23.976Hz. I have been unable to get it to output 60.0Hz or 24.0Hz as I could on Windows 7. If you are using VideoClock, it doesn't
really matter, but I would much rather it output 24/60 rather than 23/59.