This is not a direct answer to the question of GlassDoor reliability, but I have found negative GlassDoor reviews something interesting to bring up in interviews.
For example, I interviewed at a company that had a fair amount of negative reviews on GlassDoor, as well as some positive ones that came across (in my opinion) as astro-turfing. I brought this up with the guy doing my interview, and it lead to an interesting discussion about the internal dynamics of the company and what the real pros and cons are. His answer was basically that Division X had some real issues but they were working on it, but this position was going to be in Division Y which functioned in a different way. I ultimately decided against this offer for different reasons, but I appreciated the candor of the interviewer when I brought it up.
The usefulness of GlassDoor probably decreases as the company gets larger. I doubt the reviews of, say, Google or IBM have any bearing on reality because they're so massive and the experience inside the company is certainly not universal. Likewise, it's probably not that accurate for tiny companies.
I'll close with a (guarded) anecdote. I happened to know from insiders that a particular startup (~100 employees) was very dysfunctional and the leadership had severe issues. The GlassDoor reviews had detailed scathing review after detailed scathing review for a little while, then switched to entirely things along the lines of "[ThisCompany] is great! It's sooo great! And I love the leadership, especially [CTO who is a known jerk and known to be driving the company into the ground]. He/She is tough but fair and sooo smart!" So, take that anecdote for what it's worth.
For example, I interviewed at a company that had a fair amount of negative reviews on GlassDoor, as well as some positive ones that came across (in my opinion) as astro-turfing. I brought this up with the guy doing my interview, and it lead to an interesting discussion about the internal dynamics of the company and what the real pros and cons are. His answer was basically that Division X had some real issues but they were working on it, but this position was going to be in Division Y which functioned in a different way. I ultimately decided against this offer for different reasons, but I appreciated the candor of the interviewer when I brought it up.
The usefulness of GlassDoor probably decreases as the company gets larger. I doubt the reviews of, say, Google or IBM have any bearing on reality because they're so massive and the experience inside the company is certainly not universal. Likewise, it's probably not that accurate for tiny companies.
I'll close with a (guarded) anecdote. I happened to know from insiders that a particular startup (~100 employees) was very dysfunctional and the leadership had severe issues. The GlassDoor reviews had detailed scathing review after detailed scathing review for a little while, then switched to entirely things along the lines of "[ThisCompany] is great! It's sooo great! And I love the leadership, especially [CTO who is a known jerk and known to be driving the company into the ground]. He/She is tough but fair and sooo smart!" So, take that anecdote for what it's worth.