I just downloaded the Fedora 9 KDE live spin. I am impressed. I tried a few of the betas along the way, but this is very polished. Much better than Ubuntu's try at KDE4.
It feels very responsive and works very well, even as a LiveCD under VMWare. This is KDE4.0 done right. I'm banking on OpenSUSE 11 having an even better build from the betas I've seen.
By KDE 4.1's arrival, I imagine KDE will be ready for prime time and most users will be able to migrate over.
But all in all, if you want a nice Linux desktop right now, Fedora 9 seems to have hit it pretty well.