How reliable is glassdoor as a metric? Personally, I would never, ever post anything negative there due to the fear of retaliation. I suspect truly bad companies never get their comeuppance because at the end of the day, you need some kind of reference and positive job history from your previous employer. It may not be that hard to trace a review to a specific employee due to timing, title, salary, etc information.
According to the dozen seasoned job-hunters and recruiters that I know, Glassdoor is absolutely rotten in my industry (biotech). The selection bias is heavy-handed at best. Even Amazon's 5,000+ reviews represent a very small fraction of it's employees globally. These are futher selected as the kind who would review a job online pseudoanomyously.
Furthermore, one doesn't know the age of a review -- is this a current employee, a previous one who left last week or a review of a the job from 10 years ago? It's entirely possible that every problem addressed in a negative review has been since addressed. You just don't know.
I've seen plenty of instances where the Glassdor listings were really a red flag. In these cases, there are several negative reviews focusing on similar cons, then several other glowing (and obviously planted) reviews to combat it.
