I have mostly used KDE in the past but version 4's rough edges have put me off. GNome doesn't have all the flexibility I want. Now I am using Openbox (a window manager) with helper applications like Tint2 (panel) and Parcellite (clipboard).

At first, Openbox looks ugly but once you get it setup a very beautiful and appealing environment waits your commands. Openbox's speed is impressive with login timing of 2 to 3 seconds while GNome and KDE takes 30 to 50 seconds. My applications have more resources available for my use and not the desktops.

Although it may not appeal to new Linux users, I find it to be a great X window environment.