Pluspunten
Microsoft is a large company that offers a huge diversity of opportunities and experiences. You can work on traditional client applications, vertical businesses like TV and Mobile, as well as Web Services that scale bigger than or as big as anything in the market. Microsoft is generous about allowing you to learn, and fail in the process of developing your skills. It has a multitude of brilliant people that act as mentors and coaches throughout your career. There are also the traditional good reasons to work at Microsoft. Excellent pay, benefits, facilities, worldwide audience, food, transportation etc etc. Microsoft is like a university in terms of opportunities and learning. You will get out of Microsoft what you put in.
Minpunten
Microsoft is a large company, and generally the downsides are very similar. With such a large audience it's often difficult to do things quickly. I view this as more of a challenge than a downside since there is plenty of opportunity to have a large impact, you just need to know how to make things happen.