and the best part is you can't change your password in case some website becomes compromised and the hash is leaked. (without changing the secret key and as the result changing all passwords on all websites).
"If a generated password is ever compromised, you don’t need to memorize a whole new secret key and update all of your passwords. For that service only, just add an incrementing index to your secret key. For example, if your key was bananas, just use bananas2. If you can’t remember which iteration of your secret key you used for a particular service, simply try them all in order."
In particular, you don't have to use the same secret key for all websites. It's okay to slightly modify one if that password is compromised.
My bank requires me to change the password every 3 months or so and I only need to change on digit in what I remember and they see a whole new password.
It's a great benefit to me and to overall security.
How secure is it overall? Somewhat more secure than just using a single password for all sites and better trust compared to using a cloud based password storage.
Hopefully this happens infrequently enough that it's a non-issue anyway.
The extension doesn't force you to use one master password, and it doesn't have to be a dictionary word, too.
I think adding options like an auto expiring password (adding the month or year etc as a salt) can improve this further. But I like the idea in general.