KDE is one of the few environments that actually works with my setup of four monitors in a dual twinview (xinerama) configuration. Unity and GNOME3 do not work at all with this setup, they render only on half the screens, the mouse doesn't work at all, and other problems.
Currently I have to run a bastardized mix of XFCE and OpenBox to get everything to work because the XFCE window manager doesn't work correctly either. MATE (GNOME2) desktop seems to work and I have been thinking of switching (back) to it but it seems kind of buggy. It will probably end up being what I use though.
But on topic, I would love to just use KDE because it works right out of the box without me having to tweak or worry about anything. BUT, it's just too weird and often has annoying bugs/crashes (sort of like Opera actually). It looks weird and doesn't work like I think. I can't really explain exactly what it is other than "weird". It feels confusing and hard to use. If I could pick one example application that showcases the weirdness of KDE it would be the Amarok app. Good grief that thing is bizarre. The UI is so funky and doesn't work anything like what I need. For me that app is a good reflection of KDE as a whole. Bizarre, ugly, and unintuitive UI. I can't get any work done in that.