I had downloaded this funky mp4 video that was encoded with x264. The only thing that would play the vid was VLC, but even VLC was spotty, where sometimes the video would drop or just plain freeze. This file was was pretty mucked up from whoever encoded it using x264.

I tried to re-encode the video with MPEG Streamclip, and for the first time ever, Streamclip was unable to open a file for me (said it wasn't recognized).

I then turned to ffmpegX. I was able to open the file, and then re-encode it to an mpeg4 in an AVI container. The video plays beautifully now using VLC and the double pass quality that I used for encoding looks superb. The encoding was very very fast as well.

This is soon to become my favorite encoder, but I still like and will continue to use MPEG Streamclip. There's an excellent forum dedicated to this encoder as well.