I'm extremely satisfied with this downloader. Note: if it ever stops working, it's usually because Youtube changed something. DON'T WORRY. The developer works hard to update the program and always gets it working again quickly.
Also note: on rare occasion the download button doesn't appear. Just refresh the page and it should show up (always has for me).
Note 3: I've never been able to download content you have to pay for on Youtube, so I'm sure that's just not an available feature for this extension. Please don't get discouraged and give the developer a bad review if you find you can't download content that's behind a pay wall.
But yeah, this downloader is easy to use, and downloads quickly. I have absolutely no complaints.
Oh and, lastly, I've had this extension for around a year and have had no virus threats. Many extensions out there are iffy, security-wise, but this one seems completely safe and reputable (according to various reviews I've read on mozilla, and other review sites, and according to my own experience.)