This is a very good extension. I used to use (still do use it) "Disable html5 autoplay" but it wouldn't work perfectly on youtube, only on channel pages and whatnot. This extension stops autoplay on youtube completely.
However sometimes with the little testing I did there was a one second delay before it stops the video. Take a random video on youtube, refresh it a couple of times, you will see the video attempts to play but this add-on stops it. It isn't totally perfect, which is why I'll give it 4 stars. However it's much better than the other aforementioned add-on, that one works on some videos only. It's better suited for other websites, such as the annoying websites when you try to read an article a video starts playing!
This one's the best for youtube. Now I don't have any data cap problems (I'm on cable), but I still don't like autoplay videos! Let me choose when to start my videos! I will keep using this add-on.
Edit: What am I talking about, it's worth five stars. It's really a non-issue. You don't notice it when you don't refresh.