KDE 4.6.1 Almost Perfect
When KDE 4 was released, I hated it. It seemed a lot of my favorite customizations had changed, moved, or been removed. It was heavy and a resource hog. It didn't seem to work real well, things were either slow and crashy or didn't function properly. Subsequent updates did little to help. Until 4.6.1. I think KDE 4 is finally maturing into a stable and usable interface.
I got a new computer sometime during KDE 4.3 or 4.4 with a Quad Core processor with eight gigabytes of RAM and an NVIDIA 9800 GTX. Even that wasn't enough to run KDE with desktop effects and the new search engines. The first thing I didn't upon a fresh install was disable those.