Pros
Creative freedom and respect for creativity, very smart people some industry leaders that came over from other companies, building software to explore, organize and simplify the frontier of online advertising, free snacks and drinks, fun environment with jokes and laughing (if you want), parties and activities if you want to participate in that (not mandatory), world class location, good pay, company growing 2x every year, good benefits, you are not condemned for a failure or a mistake because the company is doing so well there's margin for experimentation and exploration, working with Big Data and coming up with solutions to scale things massively, anyone can initiate a project and if you can deliver on it (however small) or demonstrate its efficacy you usually get the green light to go ahead with it. If you are creative and need a place to demonstrate it, this is it. The CEO is beyond words smart about this industry, inspirational and is really nice. The rest of the upper management is top notch too. Context: I'm a C++ programmer but I also do an equal amount of coding here with Perl, Node.js, Lua and Javascript. I use the scripting languages to quickly demonstrate proof of concept and then if necessary use C++ to code a faster version.
Cons
Sometimes a very free atmosphere is difficult for some people who want to be "managed" (not me). My part of the company, engineering, is more about spontaneous projects that fit within our domain, initiated by smart people and guided by a light touch of management. As with all companies there are pockets of imperfection. People are people and that always involves a bit of friction, especially if you are working with a motivated, self-confident, smart and ambitious crew.
