I use Zamzar the same way. I use it to download files from YouTube and TeacherTube to play directly from my hard drive at school. I was a huge fan of HyperStudio and HyperLogo until it was unsupported under OS X Intel machines. I ended up moving on to MediaBlender, so I have not upgraded to version 5.0. Let me know how you like it if you upgrade.
I have used Zamzar successfully converting very short (15 - 30 sec.) Quicktime video files to mpeg. Great that you don't need to download software, but limited by upload speed. You've probably forgotten about this post - I just noticed the date!
Best regards, Britt Gow