On paper, it's great. You select the videos you want to delete, wait some time, then BOOM; all the videos you would have otherwise had to painstakingly delete yourself are gone.

However, after running some tests, I've discovered that the extension only works as intended if you stay on the tab the entire time. I was initially under the presumption that I could run it in the background while doing other stuff on my computer, but that's not the case. It will skip/forget to delete videos if you do that, so unless you plan to not be on your computer for a while, expect yourself to be staring at that progress bar.
If you don't, well, don't expect it to even perform well with a load of 10 videos; it will find a way to dampen your expectations of quickly getting that playlist under control.

My review was initially 3 stars until I saw the dev's response to CafeHaine:
Curb your attitude, pal. I wouldn't consider immediately resorting to putting customers under the bus as good PR. Your message to seitek also rubs me the wrong way. The extension has some legitimate issues that I ran experiments on, and they line up with what they may have been experiencing.

P.S. For the record before you go saying "what kind of experiments", I tested the reliability of the deletion system when monitored and unmonitored. Unmonitored, it consistently failed to delete even half of the 10 videos I would give it, whereas monitored, it was rather successful, although I have yet to conduct this test with a 100+ group when monitored.
(I also made an account specifically to criticize this extension.)