I already have several h/x264 encoded files that I can't convert with ffmpegX. Decoding with ffmpeg produces stuttering video and occasional artefacts, decoding with mencoder produces a lot of square artefacts.

The latest builds of VLC however can play the video correctly. So the libraries that can decode this should be out there. It would be nice to have improved h/x264 playback in upcoming versions.

PS: The files mentioned above are in a mkv container and all have variable framerate, so matroska support may also be an issue.