I've been trying out KDE-Unstable in Fedora 17. I'm very pleased. Rex does an awesome job with this. Some info on setting up can be found here if you are interested.
It's a great way to keep up with new versions of KDE.
Not really specific to the packages in KDE-Unstable, but my favorite feature since coming back to using KDE are activities - I just really started using them, but it's pretty awesome. I have ctrl+alt+(l,r,u,d arrow keys) to change workspaces and ctrl+alt+(l,r arrow keys) to change between the two activities I have set up. I've become really efficient with this setup.
Although I guess activities can be configured to be pretty different, I have my two set up pretty much the same, and I use them much in the same way as workspaces, but more separated. So, I have two activities with 4 workspaces each. It simply helps me keep my windows/apps/tasks more organized.
My only regret is waiting so long to give KDE a real chance again. I still have Xfce (using Kevin Fenzi's Xfce 4.10 repo) on two systems that but my main desktops will be KDE from now on.