I've hit this in Arch Linux and it's quite annoying: Open-Source GPU Drivers Causing Headaches In KDE 4.5-series. I was angry at first but then I read Martin's explanation for what was happening and decided to be more forgiving. I am experiencing this on an old system with a POC integrated video card after all; the fact that I get any effects at all is somewhat admirable.
It was pretty disappointing however that this problem exists. I didn't read them but something like this should definitely be in KDE's release announcement. Why? So my distro (who's announcements I am likely to read - after something goes wrong) can point me to it. Then I wouldn't have a) upgraded or b) had such a hard time figuring out why X was crashing, and why Desktop Effects were so PAINFULLY slow, after upgrading Xorg. I didn't ever really think it was wholly KDE's fault which I suppose helps with being forgiving AND I was being stubborn by trying to force desktop effects on through XRender after multiple crashes. After all
It worked before so why not now?
Worked there being a relative term however.
OpenGLIsSafe=true in my
$KDEHOME/share/config/kwinrc file brought me back to the painfully slow compositing, rather than crashing straight to GDM repeatedly, which I thing I tracked to kded and dbus having a lover's quarrel (YAY!). After getting the compositing back I turned off everything unecessary like transparent windows and BLUR. I only really want transparency in specific apps (Terminal, Plasma) and the blurred Oxygen window highlight.