It is a lifesaver, a few css commands and YT is usable.

Awesome - this works reliably and does not spy on me like stylus and stylish do. I would love to see it expanded to manage styles by url in the future, that's why only four stars ;)

Throws all kinds of errors for no reason, even with basic elements like #header.
It doesn't have any formatting aids either, though I suppose if all you care about is size that's not a dealbreaker.