Putting or not putting a 20-seconds spot before a web video is one of the big dilemmas advertisers are currently facing in the online world. The Wall Street Journal published a (free) article last week investigating the topic. The article quotes, among the others the opinion of a Google's executive who believes users will prefer video sites advertising free. Of course this is very much in line with Google's overall approach to the Web, and if you "translate" it, it means "don't put a 20-seconds spot before the spot, but place a contextual text-advertising link below the video".
As I user I must say I don't like the "pre-roll" ads, most of all because you cannot skip them, but I can absolutely live with them, if they grant me free access to content. Anyway, as an advertiser, instead of 15/20 seconds spots, I would rather go for 5 seconds "speedy commercials" integrated with the rest of the ads appearing on the page where the video is shown, adopting a surround sound marketing approach. Of course the more contextual to the video content the spot (and the banners) are, the higher the chances to deliver good results. What do you think?