Vantaggi
One billion of users, thousands of features, and hundreds of products. If you are smart and have elbows, you can fulfill most ambitious aspirations. Maybe you will not make difference to the universe, but you will be able to contribute to many decisions that shape industry - as part of your day-to-day work. Great benefits - anything from great medical insurance to free bus pass to posh fitness center etc. Most of all -- you are surrounded with a lot of really smart people. So if you are just out of college, Microsoft is the place to grow professionally - very quickly. If you are from industry and have something to bring to the table - you will be appreciated and respected (as long as you can control your ego).
Svantaggi
Microsoft is a very big company. And it has been around for some time. So in some ways it becomes... a bit dilbertesque. For a dozen of really good working bees, there would be one smart guy who only knows how to manage his own career. Most probably it is a "middle manager", because new hires get filtered out quite effectively. So how those career-focused people survive? - They create "processes" and "initiatives", and then you are the one to fight back. So it really matters if your boss stays technical -- as long as he remembers how to write code (and he must know it in the fist place!), you won't feel dilbert.