Pros
Big variety of projects and teams to work on, lots of opportunities for advancement, and for the most part the company is run as a meritocracy. With 20 billion in the bank, as employers go this one is quite stable -- it's unlucky you'll be laid off any time soon. There are tons of opportunities for personal growth and development, and most managers will work hard to grow you and bring out your potential. The company wants you to do well and it really shows. Of course with any company of this size there are bound to be a few bad apples, but for the most part co-workers are quite bright.
Cons
Sometimes it's easy to get lost in a company of this size. Microsoft's internet strategy feels fractured and inchoesive at times. This is a company of over-achievers and if that's not your mindset you may get left in the dust.