Regarding Rosetta:

I only recommend it for the 100 (+ or -) MB files that only play the first 10 MB. It will split the file and put it back together. I don't think it works for anything else.

For avi's that have no audio but have video (or vice versa), DivX Doctor works (usually) for those.

Some mpg's just plain won't work (as far as I know) on Mac. Has to do with the compression on PC that doesn't cross over.