Worth noting that Glassdoor is less usefull for large companies as it's essentially pay to win. It's really useful for mid-size companies though and will generally get you a good idea of what systemic problems exist within the company.
Glassdoor is only useful when there are a significant amount of negative reviews for an employer. It means that the employer is bad, so bad that they don't care enough to pay a firm to pad their Glassdoor page with glowing reviews.