I have just finally upgraded my laptop to KDE 4.2 with packages in -Current tonight. The process is very smooth if you read the README provided by Patrick Volkerding and contributed by many people including Eric Hameleers and Robby Workman.
I haven't tried all the features in KDE 4.2, but i spotted some annoyances. Here are some of them
1. Bash prompt in Konsole is not the same with previous version, so i have to override the PS1 variable
2. Amarok couldn't add a multiple directories when adding media
3. You will have to run alsaconf again in order to make the sound works again. Probably due to arts change in KDE 3.5 -> KDE 4.2
4. Lots of menus are missing their icons. You may have to rebuild the menus again. I found it easier to put all of my most used applications on the desktop rather in the menus.
5. Some application will render quite differently, such as OOo, Acrobat Reader, and many more. I think it's because of the new KDE themes.
Well, it may not be too much for small amount of time playing with KDE 4.2 since i have to prepare my materials for tomorrow.
In short, KDE 4.2 ROCKS and Thanks to PV and all of the contributors who worked hard to make it possible to enjoy KDE 4.2 on Slackware, even though it's still on -Current.
Some photos of the desktop effect in KDE 4.2 with Compiz are uploaded to my Facebook Account. It worked out-of-the-box as long as you can enable compiz in previous version of KDE or OpenGL ready.