The technology is very powerful, the people are some of the most talented I've seen. Many of the early hires set the stage for really valuable efforts -- the level of business maturity is amazing for a company this young.
Cons
Definitely long hours due to lots to do. Some disorganization and knowledge spread via word-of-mouth. The office feels like a startup -- some duct tape holding things together. All standard for a fast-growing startup, and very minor.