It works so simply and easily that I it call it "idiot proof" !! Click on the button, select the quality and size you want, and "poof" - there it is!!!

This add-on also seems to be the only one that stays with the changes of YouTube. When YouTube upgrades or alters the code for their site, this awesome add-on updates (what seems to be) instantly!! (I know is isn't instantly, but it is one of the first - and I have tried a lot of different add-ons)