I was in the same position as you a couple years ago, so here is my take.
Companies will usually have a general "trend" of reviews. In my case, I looked at a company that had a 2.0 rating and mostly negative reviews. I chalked it up to "they're just a bunch of sour grapes" and decided to join.
And you know what.....they were right. The company was horribly run and the reviews were spot on. I left the company.
My current company is the best one I have ever worked for, and it has a 3.6 rating and generally positive reviews.
People who have a negative experience are likely to write a review more than positive ones. This means that you should take a negative review with a grain of salt. For example, someone at my company left a 1-star review because they wouldn't let him bring his dog to the office, where it clearly states in company policy that you can't do that. Ridiculous. Also, people that get "laid off" due to performance reasons typically blame others.
People who write positive reviews can be HR padding the site (it happens) or when HR pushes a new person who just got hired to write a review. This means that lots of the positive reviews could be from people who aren't even engrained in the company yet and are still a bit starry-eyed.
Take all reviews of a large company (think Verizon or Google) with a grain of salt, since large companies are so diverse in pretty much every way. I would think a smaller companies reviews would be a bit more accurate.
All in all.....Don't trust Glassdoor to ultimately make a decision. Look at the trends. If the reviews are mostly negative, it's most likely not a great company, and vice versa. Ultimately, you should talk to people within the company to help make your decision. Ask lots of questions about turnover and culture in the interview to gauge how your specific team will be.
If you are in a situation where most of the reviews are negative and people are calling out the rare positive ones, I would say skip it.