The only KDE app that is worth using compared to GTK counterparts that comes to mind is probably Amarok [and I still prefer Rythmobx].
Am I missing something here?
In my experience GTK apps look and work a lot better in the KDE environment then the other way around.
For everything else, most people use the KDE version of whatever app they are looking for. I haven't used any applications other than emacs, urxvt, firefox, and amarok for years, so I can't really tell you what those are.
Edit: actually, I can tell you some things. There are themes that make Firefox look like a KDE app, and of course there is Konqueror (I tried 4.0 for a while, it was much nicer than FF2.) There is also a GIMP equivalent for KDE, but the name escapes me. My advice is to install a KDE 4.1 version of Kubuntu, and click stuff in the menus.
Well... gVim is pretty nicely integrated into Gnome, picks up theme/colors/icons, etc. I like it a lot.
Thank you for replying BTW: I will definitely install 4.1 myself. If FF and Gimp experience won't suffer, I may actually consider switching.