For the longest time I saw KDE as a distant third, or fourth, on the Linux desktop usability scale. For me, first came GNOME, then came Enlightenment E17, then came XFCE4, and then came KDE. This was a change from when KDE 3.5 — one of the most user-friendly desktops around. The KDE development team rolled out 4.0 and everything was turned on its head. KDE was no longer much use on the desktop, due to bugs, poor performance, and instability.

More information at: Linux.com