Well, I’ve been a few days with KDE 4.2 and thought I’d give a little insight of my experience and why I made the decision to go back to KDE 3.5.10.
I had 2 crashes with KDE 4.2, not surprisingly related to the Plasma portion of the desktop. Once while changing themes, and another while changing the height of a second panel.
Speed. The desktop seemed to be quite a bit faster, thanks to the implementation of QT 4.
KWin Composite engine is very nice. However, I do with I could use mouse gestures or change the keyboard keys to my liking.
It works. I contemplated putting this as a pro, but then I considered my negative experience with previous versions of KDE 4 (including KDE 4.1).
Sub-pixel hinting enabled by default
The Cons: