KDE4, much like FF3, is likely to become huge and stable in a few months. But at this point, I don't recommend using KDE4 (at least; I'm sticking to FF2 as well, but that's me).
Consider it a work in progress; it'll likely be a better experience once KDE4.1 hits, of which they've released an alpha recently.
I'd recommend you stick to KDE3 for the time being.
If you've already installed from the Kubuntu KDE4 remix live cd, do this to migrate to the normal KDE3 distro. You may want to do this in recovery mode, but it shouldn't be necessary.
$ sudo aptitude install kubuntu-desktop kubuntu-kde4-desktop_ kde4_ kde4-core_ kde4-devel_
$ sudo dpkg-reconfigure kdm