The latest DIVX codec is quite fast and efficient, and I haven't had any problems with it. They have eliminated the stupid verification process that you previously had to do to get it to work, so it's now pretty easy to use. If you want to use quicktime to play DIVX then you will probably want to play full screen:
Compile this script in script editor, then turn on the scripts menu in the menubar and put this little bugger in as a compiled script, then just select the script from the menu to play the movie full screen:
Code:
tell application "Quicktime"
activate
present movie
end tell