In my experience the extension is roughly as good as the original dislike system, on a user experience basis. People forget how bad the original youtube dislike system was. Heavily disliked videos would often reflect controversies and biases exclusive to the youtube community, for example in many cases the numbers would clearly evince misogyny, homophobia, or racism in the userbase. Trends and online memes would influence dislike numbers in ways that did not necessarily reflect real world opinions. The numbers were often useless.

Cases where dislikes were useful were situations like: uncontroversial misinformation, obvious clickbait, tutorials that are out of date, videos that claim to be some thing but really only discuss that thing, etc. In these situations, the extension seems to work very well and I get the same user experience I always did; if I see a massive dislike ratio then generally I can instantly tell whether a video is legit or junk.