I love this extension. Its got a couple big features that are so well done that I forget that they aren't just a default feature of youtube itself, like the mini-player that pops up when you scroll down to read the video description / comments.

One small feature that I wish it had, although I don't know if it would be possible, is to have each video you click open in a separate tab by default, instead of having to middle click than manually move over to the new tab.