On Ubuntu and have tried KDE, GNOME, LXDE. I don't know if OpenBox can be considered one or not, but I don't think so since it's just a window manager

LXDE is pretty quick and light for a DE, in fact I'd say the lightest ever, but I haven't had the chance to try XFCE yet because I'm too lazy to install the xubuntu-desktop package just to get it. But later I decided I don't need LXDE because later I decided it's enough if I run XTerm and loading it with Metacity/Compiz and the GNOME Panel if I wanted something very light

GNOME was my very first experience in Ubuntu thanks to class. It's a pretty neat workspace to work in, highly customizable and stuff. While not as flashy as KDE it does have its own merits for some other people I guess, because I think I just have poor taste in customizing GNOME to my own liking

KDE for me is the most visually appealing and is the reason I keep it together with GNOME. That's all. Oh, and because I'm learning to use the Qt Designer to learn Qt Ruby (now don't ask me why don't I just go to learn Qt 100% or any other language, kthx

)
I guess when you've spent some time between both it's hard to decide which one you like the most.
EDIT: Oh yeah, I might be late... but for KDE users who recently updated to KDE 4.5 and are finding it to be very slow or is a bit lagging, try disabling the "Blur" desktop effect. It was the culprit of the lag for me though, dunno about others.
This post has been edited by FlameReaper: Aug 17 2010, 06:36 PM