Just installed this extension after trying a couple of similar ones.
I decided to download this one because, besides good ratings, the author answered to some reviewer something like this: "Sorry, but your "review" is nonsense...", so I decided to use it because I like straightforward speak in an age of political correctness.
This extension works great. It gives you a button that looks like it could've come from YouTube (I mean it's not some blinking fluorescent red gif monstrosity) that, when clicked prompts you to choose whether you want audio (mp3) or video (mp4). Upon choosing it converts the video and offers you to download (or: Share, Save to dropbox, Listen, Convert another video).
Completely free, though if it continues to works this smoothly I'll donate.