Works really well in theory, but as others have said before, it puts a huge watermark on the downloaded video, then asks you to buy a "premium" version without that anti-feature.
The add-on does not tell you that beforehand, not even in the description! Downloading a video can take a long time so it is deliberately wasting your time just to increase the chances of you paying for it. I call this misleading and a shady business practice.