Far and away the best blocker I've ever used. It's extremely lightweight, it makes pages load faster, it's simple to use and configure, and it's effective at what it does without breaking websites. On top of all that, it's completely free and open-source.

There really aren't any downsides I can think of compared to other, similar extensions. This is always the first extension I add on a clean installation of Firefox, and I recommend it to everyone who needs an adblocker or element blocker.