Pros
Experience working for a large enterprise and the usefulness of having Microsoft on your resume. Also experience working with a few very talented people. However, Microsoft is a very developer led company where user experience is not well placed to make big impact on the product or get big rewards from execs (because it is seen as a peripheral discipline not a core discipline). If like me, you had great reviews and got great bonuses them you will be frustrated by the middle managers above you who usually do not come from the discipline and who you have to try to work around rather than with. Ultimately it is a frustrating place to work if you believe that design and usability should be a major driving force in product development, rather than resources that get called in when the teams think they need them (which is almost always too late). I got out because my ambitions could not easily be met in the lumgering giant that Microsoft has now become, where so much time is wasted on products by poor integration of disciplines such as program management, marketing, product management, development and test that once design and usability get involved too many bad descisions have already been made. If you want to work in a company like Microsoft was, find a medium sized company that is up and coming - a company that is agile and a company where you can sit in the room with the people who run the company (or at least your group) and who can recognize your good work when you do it.
Cons
In the User Research discipline, Microsoft is not very forward thinking and the user research groups are poorly organized and are not well placed for impact