Over the weekend I took the big plunge and upgraded my Gentoo 3.5.10 to 4.2.4, which is in testing (thanks to the Gentoo folks for making the upgrade so easy). Once it looked like it was all good, I removed all of my KDE 3 apps and libraries.
FWIW, I have to say that I am very impressed so far. I had compiled my own SVN pulls of KDE 4.0 and 4.1, but (not to beat a dead horse) they really just weren't that usable. With 4.2.4, I haven't had any crashes yet with my main apps (Konqui, Konsole, and Plasma), and the speed has been noticeably better than than the previous KDE4 releases.
I've definitely experienced the "confidence shift" and am now really looking forward to future KDE4 releases. Thanks to all the devs for your work.