After having an awful experience with KDE 4.5.0 (described here), I returned to it, as the performance issues were resolved (as I said here).
However, after that experience, something has changed in me...
I was a KDE lover. I found it way better in anything else. But after that...
I've actually been using them both. Kubuntu (with KDE 4.6) at work and Ubuntu at home.
The main point is: KDE's flagship technology is Plasma, which is the desktop, the panel and their widgets (and a few more things). The widgets... All I use is a single panel with a Kickoff menu, a task manager, a pager, a notification area and a clock. Pretty basic, I know. But that's all I need. No desktop widgets. Just a clean desktop and a simple panel.
Kind of, KDE 4, with all it's technologies, is like an overkill.
On the other hand, I never liked GNOME too much. But it gets better with DockbarX and Gnomenu, though ;-)
I must say I'm currently not 100% satisfied by neither KDE nor GNOME.
However, a new kid on the block is getting my attention: Canonical's Unity. The current Maverick netbook interface sucks. It's very slow. But the one shaping up for Natty (to be released in April) is the one I'm looking forward. I've tested the Unity 2D in the current Ubuntu, and it's very fast. And has everything I need: a simple panel and a nice launcher / task manager. Besides, it's beautiful.
Unity has a long way to go, as there's are still 1 alpha and 2 betas before the final launch. I'm really, really anxious to see how Unity will behave as my main desktop in a few months...