Well KDE4.1 is available, and it is a sweet. For a lot of people, this will be the switchover point, the rest of you – KDE4.2 should do it 
I would like to suggest looking at Bruce Byfields excellent review for a good idea of who should be switching yet.
I, I’m sold. Despite the slowness of my NVidia card natively, I can work just fine using XGL. However, I also found that XGL conversely made my KDE3 installation almost unusably slow (ditto KDE3 apps I still use – even though inside KDE4 I can hack-launch them to run on the native screen with a few tweaks).
So I split out XGL to only run for KDE4 for now. To do so I rm’d /etc/X11/xsession.d/98xserver-xgl_start-server
then created a new script in /usr/bin called startkde4-xgl with the following content:
#!/bin/bash
GL_START=/usr/share/xserver-xgl/Xgl-session
XGL_DISPLAY=:1
XGL_OPTS=”-nolisten tcp -fullscreen -br +xinerama”$XGL_START $XGL_DISPLAY $XGL_OPTS –execute /usr/lib/kde4/bin/startkde
Gave it execute permissions and added a session file for it in /usr/share/xsessions.
Please note: These instructions arethe short-version and they are for kubuntu, other distros handle XGL in very different ways.
Either way, it’s great to finally have KDE4.1 final installed – now to go PLAY !
