In my experience, and from the people I've talked to; in-browser password managers are useful only when they work without messing with. People don't rely on them, they just use them when they autofill.

The difference being, if you're not relying on them, then you're probably not actually using them to generate unique passwords on each site. So all its doing is remembering the few different passwords you use across many sites. Its a convenience tool, not a security tool.

The one exception might be Safari's, which is without a doubt the best in-browser password manager of any of them. It proactively generates new passwords, and will INTERRUPT the user from entering their own password in order to use one it generates. That's huge for average users.