I would never use Glassdoor for evaluating a company. It skews towards people who are upset, they don't verify that you actually worked there, and they combine hourly workers with salary workers, which could be two very different experiences.
Not to mention that most of the experiences are team-based. In any medium/large size company, you can end up in a terrible team and your view of the company is very negative. You can end up with a brilliant manager and great team, and you love the company.
I used to work for a company that struck a deal to include Glassdoor data on some pages we made. Then that company soon became one of Glassdoor's "best companies to work for". Which was, I'm sure, a total coincidence.
There's a lot of customer service reps in that rating. I have no idea what it's actually like over there in customer service, but over in engineering a lot of us are loving our jobs and can't imagine wanting to leave.
I actually brought this up in my interview, and asked extra questions because of it to be thorough. It's a really great company to work for, so I would say that Glassdoor's rating doesn't reflect what I've seen.
If you filter for "Software Engineers", it goes up to a 4.1, FYI.