I have Media Center Edition. It's XP Professional with a few little differences:
- The major difference from XP Pro is that you can't join domains, but there's a registry tweak to change that.
- Support for Media Center Extenders. Basically, you can set up a Media Center Extender (a box you can buy or an Xbox 360 will function as one) to stream stuff to your TV.
- Media Center Program:
Basically, it's a nicer GUI of Windows Media Player. It doesn't really do anything that Windows Media Player can't do except you can set it up to use a TV tuner.
And the looks are not any different besides the fact that the Taskbar's color is a little darker blue by default... But note that there is also a hack to give XP Pro/Home the Media Center functionality. I think it's kind of complicated though, I don't know what it is so I can't say.
Personally, I barely even use the Media Center functions. I still listen to music in iTunes, and I still watch movies in Windows Media Player. The only time I use it is to stream movies to my TV using my Xbox 360.