Enhancer for YouTube provides many convenient features, such as pausing auto-play videos by default, mini video when scrolling, auto selecting video quality among other options.

The only problem I have been having is that the various themes will make the ticked checkbox in the "Save to..." playlist menu appear Black on Black.

I definitely recommend this extension to those looking for additional customization of YouTube. If it were not for this small Save to Playlist issue I would give 5 stars.