In my opinion, online videos are a revolutionary tool for language learners and teachers. Never before have we had such access to authentic listening 'texts'.
I often use short videos in my lessons; if the video is short, we can watch it several times and take all of the good language from it. However, this doesn't mean that you shouldn't watch longer programmes or films. If you watch a longer video, don't worry too much about understanding every word or phrase; just try to follow the overall message.
Here's a longer documentary that I found interesting: