Glassdoor has a following - a VERY LARGE following.  Job seekers all over read company reviews on Glassdoor.  In fact, I have worked with several people who were considering companies and then either crossed them off the list or walked away based on Glassdoor reviews.

Why is Glassdoor so popular?

Nowhere else to gripe

For a long time, companies (aka management) ignored reviews of itself.  But employees loved having an outlet for posting their opinions.  At first, Glassdoor may have gained momentum by employees who didn't have any other outlet to share their opinions at their own company.

Whistleblowing without repercussions

Glassdoor is also a way to behave in a way employees wouldn't at work.  They don't want tell their managers what they really think of them.  But they are pissed off.  And they wanted to complain without repercussions.  Users can anonymously vent their frustrations.

But even though many companies do provide ways for employees to give feedback, the first stop is still Glassdoor.

The question is - should a job seeker make decisions based on Glassdoor reviews?

No.  I wouldn't completely cross a company off based on Glassdoor reviews.  Here's why:

1.  The employees may be disgruntled (that is why people usually complain) and looking for revenge.  But you don't have any idea whether that employee deserves to be disgruntled.  There is no way to validate their story.

2.  If a company has 10,000 employees and there are 100 posts (only 1% of the company) and they are all negative, that is still 9,900 people who DID NOT POST and might be completely happy.

3. Why not find out for yourself?  Going through the interview process will shed more light on the situation.  You will gain more information and have a better sense for whether the reviews were valid or not.

This doesn't mean you should ignore Glassdoor.  

Many companies have solid reviews from very reasonable - and happy - employees.  I searched a company I used to work for.  The company has 3.5 out of 5 stars from 1700 reviews.  With 18,000 employees globally, that is a little less than 10% of the employee population.  Would I make a decision to nix an interview for average reviews?  Nope.  Because 90% of employees haven't shared their opinion.  And that is too many to ignore.

If you've got 5 companies you are interested in - all with positive reviews - who want to interview you, then go ahead, cross the bad one off the list.  Otherwise, if the company does have bad reviews, do a little more research.  Ask around.  Try and talk to a current employee and see if the reviews are valid.