I've been looking for a good MPEG editor for MacOS X for about a year. Not found one yet.

If you want a visual editor, there is MissingMpegEdit, but it is a bit of a messy solution (it's an Applescript that combines command-line utilities with Quicktime player) and everything needs to be just right or it chokes.

Since I have a Windows box available I've given up and resorted to VideoReDo recently and despite the ugly interface, it works well.