Feels much more like big company than they'd lead you to believe in the interview process.
Pros
Very smart co-workers, fast-paced atmosphere, great networking (strong ex-employee groups after you leave), constantly changing and evolving. Mark is a strong product leader. Ability to leverage the experience for more fulfilling opportunities after you leave.
Cons
Very difficult work environment - extremely political, it combines many of the worst elements of a startup (uncertainty, lack of structure, cult-like worship of leaders) and a big company (hierarchical, hard to advance, lack of trust and needless internal competition). Large amounts of nepotism around promotions. A general culture of fear - middle management is terrified of senior management. While very smart, your colleagues will also be extremely arrogant and willing to do anything to get noticed/promoted. In general, this is not a place to go to make friends or feel like you're working together as part of a team.