edit, 22 Oct:
Design update needed since YT recently overhauled their player UI (buttons are now misaligned—but still functional!)
edit, 23 Oct:
All fixed now! Thanks nizioleque! Still the handiest YT addon around!

Works pretty dang seamlessly! Love the ability to add your own custom values, but keyboard shortcuts (rather than single-increment arrows + a clumsy drop-down list, which gets cut off at the top if you have lots of individual speed values) would make this even more intuitive. What's really missing is pitch control—all the other add-ons that manipulate pitch have clunky UI or fussy functionality. Regardless, this here's the best speed control add-on I've yet to come across.