I've had my Xbox One for a few days now and have already been experimenting with putting Sky through it via HDMI.
I have all my AV equipment going through my Yamaha receiver and then split into an external room (my office) and the main big screen and speakers. I'm pretty picking about quality and didn't expect the quality of Sky to be as good as it is, but in my opinion the HD picture from Sky really does look excellent going through the Xbox.
I also didn't expect to use the switching feature as much as I do, but it's great! I can quickly flick between sky and my game, or Netflix etc and it's incredibly smooth for me.
With the October update adding in MKV support I had hoped to ditch my WDTV Live, but alas this doesn't seem to be working at all for me. No matter what MKV I try to play via the Media player, it just pops an error saying 'this is not a supported encoded format'.
I then tried Plex, but again this isn't working well for me. I have the Plex server setup on my Synology NAS (a powerful DS1812+), but on everything I play there are problems. Firstly it will run the CPU at 100% on any reasonable bit-rate MKV (any movie that's say, 12 gigs or larger). Secondly, the disk 'volume utlization' goes to 100% after 15 minutes or so.
Both of these cause frequent pauses, which obviously makes viewing pointless.
The Plex Xbox app is new, and I hope that a thorough data scrubbing and disk check may help, but it seems you need a very powerful NAS to be able to transcode MKV properly.