GStreamer is a default media engine it Qt. It works in KDE as a Phonon backend, but its implementation seems to be not perfect at this moment. It become more neutral in my opinion, but I prefer other engines.
KDE has a basic multimedia API called Phonon, and whatever you, or your distro chose is the default backend for Phonon.
In Fedora Xine just happen to be the default. GStreamer often offers better playback while Xine has more diverse playback and VLC is what anybody should use as a standalone multimedia playback app. GStreamer and Xine are a joke...
I've never had any issue with xinelib (as a backend for KDE apps), I don't know what people are complaining about.
I've been using xine for years, and I've (relatively) recently started using KMS and Compiz too. And my DVD playback last weekend was flawless.
Bottom line: I have an old 9550 running with Fedora 12 and a composited desktop, and xine is apparently giving me tear-free playback both full-screen and in windowed mode. That's not "crap" in my book.