Some interesting research from TubeMogul shows that a sixth of users click away from pre-roll video ads; unfortunately the research doesn't comment on how long the measured ads were, or if there was a variance for different lengths - my own experience is that thirty second ads are a real problem as pre-rolls.
But the real issue is that video ads are increasingly be shown alongside video content within MPU slots; worse still, as ads are booked according to tracking cookies, media agencies see no value in contextual content, they simply identify a potential target and follow them around the web.