Pros
Absolutely the best and most brilliant people to work with in the area of "systems" research. Awesome technologies. Diverse areas of computer science to learn about and contribute to. Support for attending academic conferences. Rock solid products (routinely we have customers who run our ESX product in production for 3+ years without downtime). A tremendous amount of support in the ESX teams for innovative new ideas (including a yearly internal innovation conference that you can submit papers to!). The company has traditionally taken really good care of the employees and has enjoyed an exceptionally low turn-over rate due to this reason. The usual engineer-friendly perks like games/sports/well-stocked kitchen/gyms. The campus is very beautiful.
Cons
Growing pains associated with doubling the size of the company in just the last year and a half. Compensation for people who have been here a while hasn't kept up with industry averages (not a problem for newcomers). Promotions to Senior MTS, Staff Engineer and beyond are worse then pulling teeth. Inconsistent promotion criteria between groups. First level managers are predominantly novices (first time managers). Some recent churn in upper management has lots of people concerned but I believe it will work itself out.