Yes, I would pay for it, at least for my desktop. I see a huge benefit there, in it's resource usage.
My notebook, I don't know yet. It has the power (& programs) to do whatever I need to do with Windows 7, that one I'll have to decide upon after RC. Too early to see any benefits, not to mention that I've yet to figure out how to regain my "hotkey" functions.
From what I've been reading, on forums & other articles, as well as my personal experience, those with low-spec devices will benefit the most with Windows 8. As the months goes by, and more & more users reports their personal experiences, we'll have a better idea. Anyone who is considering the upgrade should at least tryout the RC, when it's released.