Back in the first quarter of 2007, a close friend of mine and I started a blog..
It was a niche blog for men. You know, cars, women, sports, tech stuff, drinking, games etc.
We only uploaded around 20 videos to youtube.com.
These videos were related to our niche and included our url at the bottom left corner.
In only 3 days after uploading the videos we started to get 200+ daily unique visitors.
So, it's the greatest way to get traffic.
But.. If you really want to get the best out of that free traffic, you should presell using that video.
I mean it should have some valuable information which will impress the potential visitor, it should make the audience say "hey, this guy knows something". That's my suggestion.