My experience with Glassdoor is that there is a significant amount of non-disgruntled employees reporting. I think this is because if you want to look at their data, you have to put in some details about your own employer. Certainly most people are writing either milquetoast feedback that gets them through the door or moaning about their employer so the information is biased, but it's not quite Yelp. Either way, Apple has 154k employees, it's such a large company that it'd be surprising if in aggregate it was the best place to work.