As with all flashy (pun intended) web technologies, they can be effective if used with good taste and in moderation.

For example, the Path landing page uses a well designed background video and falls back to a simple image if the video isn't available: https://path.com/

(Incidentally the background video didn't load for me every time. Straight link to the video: https://s3.amazonaws.com/assets.path.com/v2_home/background....)