For its time it was a great design that was intuitive to understand, relatively lightweight and did not get in my way. About the only changes I think improved things notably were the search field in the Start menu and Aero Snap.
So, it's also an ad delivery machine.
Cortana won hands down by actually telling a joke (it was about dirty laundry, but it was ambiguous enough that I think Cortana actually "got it".
OTOH, I feel really weird literally talking to a computer, and I don't think I will ever overcome that.
I only use Windows at work, and my work computer is sufficiently slow that I have not had a problem with that. Maybe it is the version, too, my work laptop runs Windows 7 - and will hopefully continue to do so until Microsoft stops supporting it; as far as Windows goes, Microsoft really nailed it with Windows 7. (I have to admit, though, I have only very little experience with Windows 10; I really disliked Windows 8/8.1 and the corresponding server versions because Microsoft butchered the UI.)