I have talked to a fair number of Foursquare employees, and know a few others from outside Foursquare. It really seems like one of the less dysfunctional tech companies, and a bit smaller than I thought.
The only negative anyone ever mentioned isn't really a secret, and some people consider it a positive. They have always been pretty open about not wanting to sell the company, which might be great (Facebook was the same way), and is fine if you already have money, but reduces the chances your equity will be worth $100k to $1mm or so; it may increase the expected value (due to a lower probability but huge success event). It does mean the company is less likely to get bought by a company which marginalizes the product and team, too.
The only negative anyone ever mentioned isn't really a secret, and some people consider it a positive. They have always been pretty open about not wanting to sell the company, which might be great (Facebook was the same way), and is fine if you already have money, but reduces the chances your equity will be worth $100k to $1mm or so; it may increase the expected value (due to a lower probability but huge success event). It does mean the company is less likely to get bought by a company which marginalizes the product and team, too.