KDE 4.8 works well, but a few anyone things like trying to open a konsole from the menu never opened on the active head. Composite would sometimes just stop working on one head, but continue to work on the other heads. A kwin --replace would fix this issue ( after several attempts ).
My biggest complaint with KDE is my computer just felt slooooooow. for a quad core with 12GB mem. Even with composite disabled, things felt slower than what I had been used to from running GNOME 2 for some time prior. Slow login and even after login I would launch a Konsole icon and sometimes sit and wait for 20 seconds while KDE continues to initialize or do something. Alt tab between apps, min/max windows and general paints just seemed very sluggish. Not a scientific benchmark, but spending years and hours in front of a Linux desktop, and you just know.
I switched to XFCE4 / Slim packages a few days ago and couldn't be happier. I feel like my computer is back again. Very fast, and it understands the 3 heads very well for placing Panels where I want them placed. And the Terminal Emulator menu actually opens the terminal on the active head with mouse focus. Compositor works well, though not as feature rich as KDE, I get nice transparencies and shadows that work.
I noticed xfce4 tears playing video in VLC more that KDE when compositor is enabled, but a quick disable of the compositor works fast and well to turn on and off which fixes the tearing. Turning composite on/off with the KDE system caused issues.
XFCE4 is now also on my laptop and it works very well for me.