[5-star ratings utterly dominate][link] the distribution of overall votes. 1 star votes are in “second place”, but as the graph shows it isn’t even close.
Star ratings really don’t work well on the internet. YouTube would be much better off with a simple “thumbs up” and “thumbs down” system, and doing their algorithmic magic on those.
The short answer to the question in the title of this post: not very.
[link]: http://youtube-global.blogspot.com/2009/09/five-stars-dominate-ratings.html “YouTube Blog: Five Stars Dominate Ratings”