Judging from consumer preview I can't say that I find Modern UI impossible to use, just from my limited time with it there seems to be a lot of doubling up - two different control panels (settings and control panel) illustrates these confusing instances of overlap. Modern UI seems to work, just they don't seem to have found a way to adapt it to the typical workflows of windows power users, and instead there are messy coexistence of new and old tools and menus. Personally installing a third party shell or UI is not an acceptable solution, I like to have as few services running as possible, as few programs to update, licences to expire etc.
This leads me onto my other problem with W8 - the DPC latency is much higher than W7, which is a no-no for pro audio. For audio playback, som users have commented that W8 sounds more "solid/cohesive" than W7 - I'm not sure what to make of this, but I will definitely try the consumer preview (for sound quality) before I replace my existing W7 install to decide if there is any difference, and which I prefer.
Personally I care little for small differences in desktop performance if the user interface is not refined, and if the audio performance is unknown. My system really needs a new OS licence installed at this point, I will have a hard time deciding which to buy at this point. I think in the end sound quality (if there is a difference, and I expect there will be) will be the deciding factor.