Glassdoor can be (and is) easily tainted by submitting fake positive reviews. They have a policy of not letting employers remove negative reviews, but that doesn't stop the company from flooding with "it's a fast-paced work environment, so some people clearly can't handle the heat in the kitchen" style "positive reviews" to reframe the legitimate negative ones.

So, it can sometimes be difficult to get a proper read on signal. However, it seems to be among the best data source out there.

I love glassdoor. Sometime you see high level management on there giving frank plus and minus assessments of the business, often critical of higher levels. It's not just disgruntled departures but often current employees.