Pros
Microsoft has by and large smart people. You will spend your day interacting with them, and they in turn will stretch you. People are passionate about what they work on, are vocal about it, and you will like most of your colleagues. Great access to information across the company - from other teams to the library and Microsoft Research. It is also a great place to experience different kinds of technology and you can move with relative ease across the company. Benefits are amazing. Work-life balance is highly possible. The culture is in general very inclusive.
Cons
If you are looking to make money, rise rapidly, Microsoft is not for you. There are layers upon layers of management and bureaucracy. Promotions are often handed out, not based on your merit, but on how well your manager can argue for you. Projects can go on indefinitely as management can often be clueless or lacking the spine to make changes. Microsoft is slow to react to changes in the market. If an organization is under-performing, it will be atleast 2 years before a change in leadership happens, and another 3 before the right direction is visible in the market. In the meantime, competition can have a field day.