It depends on what you want to do.
I am running Slackware 12.2 with KDE 3.5.10. My highly customized Slackware system does all I need it to do. There is simply no need for me to throw out my configuration just to use KDE4 (and spend considerable time to create a configuration that serves my needs.) I am likely going to install Trinity, to get the fixes and features they built in without the major surgery for an update to KDE4.