Pros
Really smart co-workers, very positive culture, great work-life balance, paid overtime, lots of random perks. The people you are surrounded with are highly intelligent and passionate and generally very upbeat about their jobs. This makes a huge difference day in and day out, because if you work in a negative place (as I have in the past), it can get you down. Positive, happy workplaces like this make it easy to get through the week.
Cons
Like any big organization, there are a lot of procedures that limit flexibility. There are a lot of little random policies that are quite silly and unproductive. There are no cups for water for example, you have to own your own mug or something. This can get pretty embarrassing when a client or vendor comes by, and you have to go buy them a water from the vending machine, because there are actually no cups.