MPEG Streamclip is decent for OS X for being nearly free. (needs the Apple MPEG-2 component).

Hurrah! Finally a Mac MPEG editor that seems to work with everything I've thrown at it so far. Thank you!

Sadly I still don't have an easy way of editing commercials from my ReplayTV MPEG2 files and burning them to DVD.

Have you tried Sizzle for burning DVDs? You can just drag-and-drop a bunch of MPEG2 files into it and it creates a DVD disk image with a menu.