This addon works still on YT with HD-Videos; where other addons don't work anymore, this one didn't let me down yet. The developer keeps up with updates to keep it running. As far as I got it's not possible anymore to download HD-videos directly from YT with addons, so this addon grabs it with an external server and delivers it as regular download from there.

I recommend to upgrade to the paid version, as you get the full download functionality (otherwise you can only download in lesser resolutions, OR if the HD-Version of a video already was downloaded by a pro User)
When I bought the Pro version, I had an issue with downloading videos in HD, but the developer responded within one day, and I could download everything, thank to the dev here, again :)