Pros
Working at Microsoft is awesome. If you are into technology, having the ability to influence the direction of future products is a huge source of personal satisfaction. More than that, working with the brightest people in the industry makes the creative process really rewarding. For people with great interperonal skills, the opportunities to change the world are boundless. Finally, the way that employee development and satisfaction is being managed makes it possible for everyone to get the most out of their job, if that's what they want. Other elements are also awesome such as the campuses around the world - they're beautiful, and the food in the company cafeterias is better than most chain restaraunts (way better). Oh, and finally - when you do a good job, you get geat financial benefits.
Cons
Let's see... downsides. For people who I've known who come to the company and then leave, there is a lot of "luck of the draw" in the hiring process. If you hire into a group that is not well managed (it happens), you can find yourself in a high pressure, long hour job with no end in sight. The best advice I have hiring into Microsoft is to be careful in the way you interview your next employer. Since there are so many jobs available across many diverse groups, be sure to not land a role only with the thinking "get your foot in the door. Nobody benefits when a new hire burns out and leaves in the first year.