Pros
You have the opportunity to make an impact on the world stage along side some of the best and the brightest from all over the world. Microsoft takes an active interest in developing its employees and their compensation and benefits are top-notch. Microsoft listens to its employees. The programs and benefits it offers are constantly changing in reponse to that feedback. Finally, its a company with a very diverse product line that is always moving into new areas so it's possible to make big career changes without needing to start over with a new company.
Cons
Its a huge company and despite its constant financial success its stock price is stagnant. Its not for the get-rich-quick crowd. It can be a bit demoralizing to not be able to read the tech news without finding some article demonizing Microsoft for one thing or another. Microsoft is making huge strides in improving its reputation, but it still has a long way to go. You need to develop a thick skin for dealing with outspoken anti-Microsoft zealots.