Standard videos uploaded onto YouTube are at 320×240, ≅200 kbps whereas the newer high quality versions weigh in at 480×360, ≅900 kbps. That’s a pretty significant difference in video quality, not to mention the audio quality is better as well.
In order to download the high res. video, just append &fmt=6 to the end of any YouTube video’s url. The end result is a much better .flv which you can convert it to whatever you want. Personally, I like to take videos with me on my iPhone and share it with my friends during breaks and in between classes. The need to convert the videos into .mp4 can get pretty annoying when you download a bunch of videos at once. Thankfully, there is a way to download the original high res. video in .mp4. To do this, just append &fmt=18 at the end of the url.
There’s a plethora of applications, scripts and websites that will do the download for you. All you need to do is provide the link. Personally, I use KeepVid.
Bear in mind that not all videos have a high res. version so your mileage might vary.