There's one issue that I am aware of between nVidia (?) and KDE (most likely KDE related).
Somehow the display on two of my 4.6rc machine simply refuses to go to sleep. 
In-order to solve it, I'm forced to:
- GilboaCode:$ sleep 30s; xset dpms force suspend Ctrl-Alt-L
Yeah, had to redo my power profiles because they stopped working properly.
The problem with Nvidia open source graphics is that you get no 3D support. Desktop effects, 3D games and other assorted things won't work. I'm a gamer, so that doesn't sit well with me, I always have to install the proprietary drivers for my Nvidia card.
There were a lot of problems, especially with the earlier 4.X builds. It's been discussed quite a bit on the KDE forums. The plasma desktop didn't play nice with the Nvidia proprietary drivers and, as I said above, Nvidia graphics adapters don't work all that well with the open source drivers. I haven't experienced as bad of problems as some people have (depends on your card) but I can't say it performs as well as when I used to use the Intel graphics adapter built into my motherboard.Originally Posted by gilboa
Oh great, I upgraded my Arch installation and now Plasma crashes as soon as I open any maximized window. R600c here.
Way to go, KDE!