Very good and have been using it for a long time on Firefox. It keeps the time stamp of the original video upload date which is very useful and no other app I know of does that.
Ignore the 1 star reviews saying it isn't working. This is temporary. Youtube have blocked it which it has also in the past, but it always get sorted, so be patient. If you left a 1 star review, I hope you remove it when the add-on is working again.