If your on a Mac and your using Safari then there is an easy way to download your favorite You Tube movies to watch time and time again without the hassle of downloading each time. Better yet, if you've got an iPad then you can pop them onto your iPad and use a viewer like Good Player to view wherever you are, whenever you like!
So first up open the You Tube page with the movie and let it begin playing. Next press command-option-A which will bring up an activity window. Find the You Tube heading and click on the arrow to show details of what is being loaded. Somewhere in that list there will be a large file, typically over 5MB. Double click on it and Safari will begin downloading it. Once the download is completed you can find the file in the download folder via Finder, it will be a .flv file and have a generic name which I suggest you change so that it's easier to recognize.
If your using Good Reader you can transfer the file to your iPad via iTunes. If you'd like to view on your Mac then you will need another media player like VLC.