It's ok. I get it mixed with my left/right arrows a lot so that I think I'm moving forward or backward, but I'm actually changing the speed instead. I removed it because it sits so near the bottom of the screen that I keep accidentally moving out of it altogether when I mouse over the speed control.

It works, it's not technically bad, but it's just too simple to be practically useful.

For any but the simplest conceivable use case, an extension like this needs to track speed separately per open video/per tab, and should revert to a base value on opening a new tab. This one doesn't seem able to do either.

It also causes an issue when viewing YT Shorts, as it uses the last set speed, but neither this extension nor YT itself provide a speed control. It needs to either disable itself or provide a speed control, in this case.