Pros
There are tremendous opportunities to make lateral moves within the company - get bored of a given role, org, product, or customer segment and the sheer size and internal policies conducive to internal transfers guarantee the opportunity for variety in your MSFT career. Also impact - the vastness of the customer base means that even "small" products at Microsoft often touch on millions of customers. It's rare to find an organization where even junior staff can see direct impact of their work on such an immense scale.
Cons
Size breeds laborious process and overbearing politics - likely the same in any company with over 80,000 employees. It becomes a significant challenge to get meaningful work done, as you end up spending huge percentages of your time churning through v-teams, lining up stakeholders, navigating ownership issues, and the like.