KDE has always tried to play catch-up with Windows regarding style, leaving users in some kind of uncanny valley. And I'm not very impressed with what I've seen lately in KDE. That said, I'm hopeful something useful will come out of it.
I know this sounds ridiculous but KDE/Plasma5 is great if you manage to change the style to something else. Numix + icons is a good candidate. You can choose QtCurve for KDE/Qt5/Qt4 apps and skin that with Numix + Numix Colors and GTK2/GTK3 have native Numix widgets.
I've made peace with Breeze Dark at the moment, it's not so bad.
But optics aside KDE5 is mighty powerful. Dolphin is a great filemanager and I'm missing it on Windows. On the fly ssh/sftp/webdav is also a great feature.
The Plasma NM applet for NetworkManager is also great - especially if you often change settings for interfaces.
The menu works fine, searching is almost instant. I'm using the current KDE from Arch Linux and it's pretty stable for me.
Overall I'm happy and I'm not looking at Unity - that also has it's merits but the whole Nautilus/gvfs ecosystem was kind of unstable for me. YMMV.
What I dislike about KDE are the widgets - besides the essential stuff like time,battery everything feels clunky and slow - the system for adding them is also not so great. I've also never used 'Activities' the whole concept feels pretty much out of place. I'm also setting up my Desktop as background and ignore the "folder in a plasmoid widget" story that is bullshit IMHO.
I'm also avoiding akonadi/kmail/korganizer etc.pp - these might have improved but still felt clumsy last time I looked.
IMHO it's getting better but it's no silver bullet. For most users the default presentation and UI and settings are just confusing and ugly IMHO - if you look past that you will find a lot of good, stable and well thought ideas.
I'm running KDE now on all my Desktop Linux boxes and I can't remember the last time I had some hassle with it. It does what I want, it goes mostly out of my way and it doesn't look too bad.
For all the stuff it only uses 300-400MB RSS memory after a fresh reboot with a few Konsole windows running.
And really don't forget that KDE actually has a file picker that lets you browse by thumbnail, something that has been missing in Gnome since a bug about it was filed in 2005, and it's still not there (yet).
Plasma was before there was anything similar on Windows.
But to be honest my perfect linux enviroment was kde 3.5. And I don't think "regular" people had any problems with uncanny valley. When I left for university I installed kde 3.5 on my parents' and my sisters' computers. My sister switched to windows 8 years later because of some apps she needs for work, parents still use Linux (but recently I installed them kde 5).
