Extremely useful and properly functioning addon. It might take some time to get used to the interface (like videos detected in tab, running downloads, finished downloads, etc..), but it is pretty straightforward.. Multiple downloads are possible at the same time (up to 6 or 7 simultaneous downloads, but any extra will be queued and will be downloaded without any problems.. I was able to click "quick download" on about 25 videos in a webpage and all of them were downloaded eventually with no problems).
Very useful when you have a subscription to an online video library/course that you don't want to keep paying for every month just to watch the same videos over and over again.