I have just upgraded to KDE 4.2 yesterday. So far so good. No package breaks. Standalone package like kpdf and kbabel still survive and works as normal.
The desktop is quite different. I miss the "Show Desktop" button. I don't use widget because I personally think that it waste CPU cycle.
One question: In KDE 3 I can put shortcut of Iceweasel, Icedove and Konsole on the panel, near the Launcher icon. How do I do it in KDE4? This setup is the same like my Windows XP, where I put commonly use software near the start button.