Pros
One of the biggest pros to work at Booking.com, at least when I was there, was the people. At the time, we had a pretty strong leadership.
Cons
When I left the company, most of the leaders had left the company as well. At that time, the company lost its focus. The management started to add layers and layers of bureaucracy but still called it a "flat structure." At the time, there were Team Leaders, Über Team Leaders and Senior Team Leaders, making it clear that a hierarchy did exist. Most of these weren't able to manage people (even though many of them were great developers) what led to a great frustration. I've seen plenty of team leaders step down because they weren't valued, even if they gave everything they had to help the company. In the other hand, many yes-saying people just moved to the top of the food-chain.