Since my main PC at home runs Linux, I could afford to take a gamble on Windows 10 (as it's non-essential). So I took advantage of one of the recent offers from Lenovo. After fitting an SSD and clean installing Windows 10 (thanks MS for the downloadable ISO) it runs smoothly and is a big improvement over Win8.1 (without Lenovo malware/crudware).
Despite the barrage of Windows 10 criticism on el Reg, fundamentally Windows 10 looks to be a usable successor to 7 as a desktop OS, but it can also be used in tablet mode to replace Win8 on touch devices. I expect once those remaining issues are fixed, few Win 7 or 8 users will have reason not to accept the free upgrade.
How usable Windows is depends on how you use it. Personally I think having apps sorted A-Z on the Start menu is a benefit since you don't have to spend time arranging them and you can find apps quicker on other people's PCs. Besides any app you use regularly, you would pin to the taskbar.
It wouldn't surprise me if, next time the average home user upgrades, a surface type device running Windows 10 could replace the function of both a tablet and a laptop, with the advantage of an OS that gets reliable software updates. I wouldn't have said that for Windows 8, but for 10 it's possible.