Pros
Make a difference in the lives of others across the globe. Be invovled in the cutting edge. Large company resources, coupled with the freedom and feel that individual groups have to behave like startups. Unbeatable benefits. Surrounded by intelligent people. Comfortable, casual offices and attire. Vast career opportunities. Once you're in, you're "safe" and protected from market forces. There's a process or website for EVERY possible question anyone might have--lots of resources and support.
Cons
Politics, politics, politics. Low pay. Stock is given with a 5 year vesting period, to keep people from leaving. To truly succeed, must forego personal life or time outside of work. The Microsoft vocabulary and constant stream of acronyms. Slow promotion cycles. Managers aren't expected/required to make their own contribution, but to only manage others' contributions.