lupineshadow: Windows 8 - horrible
Windows 10 - seems ok but who knows what's going on under the hood
I found Windows 8.1 decent enough, except the metro interface. Low on RAM usage and reliable all-around. Excellent for gaming. The UI was a downgrade from Windows 7, but not horrible by any means.
On the other hand, Windows 10 has tripled (if not quadrupled) the number of processes, which led to adding more strain on the RAM. Updates have been hit and miss, the UI is worse than ever (MS can't even decide between legacy and modern), it comes bundled with adware (and not only that), privacy is non-existant out of the box (instead of opting-in you have to opt-out and even then there is no guarantee it will behave as intended, you'd basically have to lock it down with a good firewall).
If you are curious enough, you could learn what is under the hood, you just have to know where to look.
I would call it the worst OS from many standpoints, it has its positives, but even those are outweighed by the negatives.
If the user does not care at all about their machine and how it works, then I guess it can be called a good OS (for that type of user, of course). If they care, even a tiny bit, they will probably realise 10 is nothing short of a dumpster fire. It gained popularity due to three factors: price (or lack of it), direct upgrade path from 7/8 and DirectX 12. Driver support is also at its best from what I noticed.
Something else I think it's worth mentioning: Windows generates 14% of Microsoft's revenue. The main focus has been cloud computing ever since management changed.