The Youtube Downloader integrates seamlessly into the YT UI, works excellently, and appears to be regularly and well maintained, always adapting very quickly to changes in the YT codebase. I’ve downloaded many gigabytes of videos using it, which is especially useful when living with slow, limited capacity internet connections.

The one gripe I have with this, as well as many other addons, is the intrusive, annoying way it announces automatic updates by randomly opening a new tab, but in lieu of a better UI for this, it’s not a terrible solution.