It seems to give different options for the video quality each time you try to download the same thing.
When you just click on a download option it doesn't ask you where you want to save or tell you where it's saving it, you have to click on the right-arrow to the right to do that.
It doesn't show you the current download status or tell you when it's done.
The default filename for YouTube videos is apparently just "YouTube.webm" or whatever extension, so if you want to give it a decent name you have to type it out or copy it from the YouTube listing.
It has a ton of options for formats to convert the video to, including at least one audio format, but somehow it doesn't have any good, modern audio formats like m4a, ogg, opus or wma.
It requires you to download and install an external application for it to work, so you don't get the safety of just running an approved and contained FireFox addon, who knows what malware, spyware or adware is in it.
Just about the only good thing about this addon that it's actually able to download videos.
Actually, I take that back.. it only downloads successfully some of the time. Usually on YouTube I get an error similar to this:

Failed aggregating "Paul Anka - Let It Snow - YouTube"

D:\mp3\d:\mp3\christmas\youtube\Paul Anka - Let It Snow - YouTube-IDGEaS6vs8Y.mkv: Invalid argument

I haven't figured out a way around that. I'm lowering my rating from 3 to 2.