I've been using this add-on for just over a year now and I can say without hyperbole it has enhanced my life. It's simple on the user-end, infinitely superior to my old methods of trying to obtain things from Youtube. No ads, no spyware, no bloat. I paid for the PRO upgrade - all of $5-$10 (cheap!) - and don't regret it one bit. Bit of advice: If you upgrade, keep that email in your inbox because if you clear your cookies, the add-on will forget you paid so you'll need to go back and click the Pro confirmation link again. My only real complaint is that many older videos were uploaded in 480p but there is no option for that resolution. Also, sometimes it breaks, but that's Youtube's fault and the programmer does his best to patch it as soon as possible.

TL;DR - Highly Recommended.