Count another happy KDE user here.
I have an unconventional setup with one big window and three smaller windows at the side, didn't need to do much to keep that layout consistent over the years. It's also set up similar to Unity (sidebar with launchers instead of horizontal) because I wrote a few things to make use of the Unity API, supported by the "Icon-Only Task Manager plasmoid", which I believe is available by default.
However, unlike Unity, my window controls are on the right side because I am used to that, the behavior when clicking on window groups is much more rational and fast (not to mention you can use the mousewheel to quickly switch), and I use either regular menubars or menus hidden in a window decoration button. The global menubar paradigm is not my thing. I basically took what I liked from Unity while keeping a more normal behavior, yet using an unorthodox layout.
Time spent configuring? Like 2 hours over the years.
KDE gives me a lot of control. I can tab windows together, from different processes, automatically or manually; have specific windows appear with specific sizes at specific places, or have them remembered geometry if the app doesn't support that, all through an easy-to-use GUI; I can give windows hotkeys to quickly switch focus to them (I use winkey+X to quickly switch to my terminal, Konsole), or disable compositing if I am developing with OpenGL. I was even able to tame GIMP's 2.5.x oddball interface back in the day using a few rules (not needed anymore though, I love the new GIMP)
It doesn't get as much love with plasmoids as gnome gets with extensions, but you can still find a few handy 3rd party plasmoids here and there.
Anyway, I am very satisfied with KDE. The first two releases of KDE4 were ass, but it got way better by the fourth/fifth release, and nowadays it's really good to work with, even akonadi behaves as it should, which I thought I'd never see the day.
As you, my second choice of desktop is also XFCE. As you say it's pretty good using few resources, behaves pretty much as you'd expect if you are used to windows desktops, and while it's a bit lacking in terms of addons, it's pretty decent at what it does. I love it on old laptops or netbooks.
I also appreciate the fact that they managed to make a tasteful "mascot" (just a mouse silhouette, it's cutesy but not too much), unlike other projects using animal/mascot motifs. I cringue every time I see remmants of that cartoony dragon that was the KDE mascot. It's not that it looks bad, but feels so out of place being so...cutesy. Looks like the dragons there were in children cartoons of the 80s.