I recently moved to a new machine, switching from Fedora Core 4 to Fedora Core 5, both with KDE 3.5.3 from kde-redhat. On FC4, gtk apps like Firefox, Thunderbird, Adobe Reader, and OpenOffice blended in pretty nicely, but on FC5 the widgets look rather ugly. I tried gtk-qt-engine, but it broke OpenOffice: all the toolbar buttons disappeared except when I hovered over them.
For now I'm brining up gnome-theme-manager manually which activates the Clearlooks theme; the Clearlooks widgets blend in well enough with my KDE Plastic widgets; I have the "Apply colors to non-KDE applications" checked which overrides most of the Clearlooks colors so they match KDE apps. But its a pain to do this and I noticed it does seem to have some side effects; for one I noticed gnome-screensaver activated rather than the KDE screensaver.
Currently using KDE 3.5.3-11.0.fc5.kde.