I have an openSUSE 11 KDE 4.0.4 install which I have been neglecting while I use Mint everyday. What can I say except that I really like Mint more? Anyway, I decided to do the one-click install to 4.1 today after reading these posts. I did not want to mess up my Mint, so if I bork openSUSE, it is not a big deal. I am quite adept at clean-installs.
My initial impressions are extremely favorable. There are certainly some massive improvements to the desktop. That horrible cashew in the upper-right hand corner of the screen is no longer hyperactive. The plasma widgets are still unusual, but more tamed. Plasma is so different from anything in KDE 3.5, and especially GNOME, that it is going to take a while for me to get used to how everything interacts. After some time in the openSUSE forum, I should have a better grasp on the situation.
As for stability, openSUSE's implementation of KDE 4 is being heralded as the best, though I have not used KDE 4 in any other distro to corroborate that opinion. I do know that everything works and is stable. Nothing I have done has flaked out or gone all wonky. There are still several KDE 3 applications being used due to no KDE 4 version.
KDE 4 is getting better, and it is ready for use on a daily basis if you do not expect flawless perfection. As soon as more applications are updated, such as Amarok, it should be an even more exciting experience.
That being said, I still prefer Clem's Mint 5 GNOME.