Works about half the time Rated 4 out of 5 stars

When it works it's great. But it's very inconsistent. For example, these videos still autoplay with the extension installed and enabled:

http://www.youtube.com/watch?feature=player_embedded&v=GCXHPKhRCVg
http://www.youtube.com/watch?v=SXam2uyd3-o&feature=player_embedded

What makes these videos different from the ones that don't autoplay, I have no idea. This would be a five-star extension if it worked 100% of the time. Or even 90% of the time.

UPDATE: Yes, sure enough I am in the HTML5 trial. Well, I still can't go to five stars given that not all YouTube videos are stopped by the extension. However, I will bump it up to four stars given the developer's quick answer and awareness of the issue. Hopefully it can be resolved, and if and when it is, I will happily bump this up to five stars.

This review is for a previous version of the add-on (1.0.1.1-signed.1-signed). 

Html5?

Are you in the html5 trial by any chance? If you right click the video, the last menu item will be "About HTML5", which is the case for both of the videos.
It seems Youtube changed something with html5 videos and broke the add-on.