I have, at a truly well-run engineering team. She was fun to bounce ideas off of, and quick to rope in relevant experts, business or techincal. She also understood the value proposition of each feature, and could easily spot if you were wasting cycles on something unimportant.
Probably not coincidentally, this was the only place I've worked where KPIs were truly a useful way of framing our place within the organization.
I've spent a lot of my time helping ours grow into a collaborating by walking them through my expectations of a product brief and what we need from our user stories. They're also really good at helping with the release, managing story sign-off and communicating changes with the other teams (customer success, account managers, sales team, etc). A good product manager manages... the product. They also collect all the user feedback, validate feature requests with data, and act as a guard for the rest of their engineering team.
