I have only good things to say about Perian, so if that would tend to bore you, stop reading!

The whole process was soooo easy,

in Firefox I put in 'player for avi in macs.
Went to the Perian option.
Read up on the blurb..liked it and went on to
Download.
Tried out several movies and TV series and all played immediately in Quicktime with absolutely no problems whatever.
Er...that is the 'Quicktime' I had tried with many other bits and pieces downloaded to try to make it work!