I do think reviews are reliable source of current situation of a company, I trust negatives ones more than the positives. Its not just for glassdoor but for any reviews that you see on yelp,google or fb. I sift through the negative reviews and try to find a common factor in the reviews, that would be a way to validate the review.

I trust negative reviews more because if I had a good experience, I don't have any incentive at all to login to a webpage, create credentials if its needed and then go through the myriad of steps to post a positive review. But on the other hand, if I had a bad experience, the only way to vent it out is by giving them a bad review. I am willing to go through the hoops of signups and surveys so that I post that one bad review about the service. There is an incentive for me there.

Tailpiece: There are many services that you can purchase to write positive reviews for your company/service. The trend to identify this is that whenever there is a negative review, it gets buried by a bunch of near 5 star positive reviews, mostly with a one liner review text.