Plasma + KWin = beautiful !
Until last week I was still running good old Slackware 12.1 with KDE 3.5.something.
I had skipped Slackware 13.0, because it had still no support for virtual screens in X/xrandr, and the KDE 4.4 in Slackware 13.1 crashed a few times while playing around with it. So I stayed with 12.1. For building current KDE, it doesn't matter what desktop you're running, so that was fine for me.
Now last week the new Slackware 13.37 was released and I gave it a try. Virtual screens are back in xrandr (now called "panning"), good.
And it comes with KDE 4.5.5.
To make it short: it simply looks beautiful and it did not crash once since then (except the kwin+Intel+mesa crash Martin blogged about) ! Effects work flawlessly and smooth.