I've been running Win8 since the Consumer Preview came out and now doing the Release Preview. I've seen some good improvements in terms of performance over 7 and a lot lower memory footprint overall. Base install also only too about 15 minutes from a USB key to an SSD.
In regards to gaming performance, it's really less about the OS and much more about the hardware drivers. I had some pretty major issues with the AMD driver release for CP. Going to the RP drivers fixed a lot of issues I had, especially with Eyefinity.
In terms of day to day use, I most of my computers have Win8 now. I use it at work and on two of my machines at home. Metro does take getting used to, and I'll admit there are some issues with mouse control, but they're not huge. I've gotten used to the changes and really like the polish that has been put on the RP.
With that said, Microsoft's target for a new OS aren't really people with existing hardware. The vast majority of computers that get upgraded are on the business side and much less in the consumer market. Most of the people who wind up with Win8 are the ones who buy new computers with it preloaded. I wouldn't be surprised to see a lot more touch-based hardware coming out this fall. Only Apple users really consistently plunk down the $100 (or whatever it is) every 2 years to get a new release of OSX.
Finally, it's a bit unfair to blame the poor performance of FFXIV (especially when it comes to the UAC prompt) on the OS. To me, that's just a lack of testing and lazy coding. Maybe it's something unavoidable, but how do other games get around it? If you really understand how UAC works, it's easier to appreciate what the purpose is. The "secure desktop" is actually a different user session than the standard desktop. Therefore, anything in that user session is completely separate from your standard login and mostly unreachable by malware or code of any type.
This seems to cause problems for XIV since it doesn't know how to deal with DirectX suddenly getting paused. I don't see this as a Windows problem, since the feature existed long before XIV came into play.