For me, I found I did not need to do 3 way merges that often and opendiff's native UI fits in better than kdiff3 (for me). I think kdiff3 was Qt? Despite Trolltech's best efforts, Qt does not feel native on a Mac.
This isn't to say that KDiff3 isn't great - it is.
I don't think KDiff 3 looks native anywhere, and I don't think it's because of Qt.
It uses weird fonts and icons, and for some reason their toolbar buttons just look wrong.
Still an extremely useful program.
[1] http://kdiff3.sourceforge.net/