Ive had some fun(!) in converting an MPEG4 H.264 on my Mac (OSX Mountain Lion 10.8.3) to an older format, namely DIVX and also XVID avi - why you may ask? Well because my perfectly good player is a couple years older than I would like it to be, and happily plays XVID/DIVX but not new enough to play H.264.
I tried iFFMPEG, nope, player didnt recognise. I tried with Handbrake - broke in the player!
ffmpegX was a success! (and after finally getting it working, see previous post) it was by far the quickest and easist to use - drag, drop, and go!
:)