Pros
Health benefits, various perks here and there. Excellent job security, especially comparied to other companies in the same industry. Very flexible work hours, you don't actually have to work very hard if you don't really want to get back much. Generally speaking, you are surrounded with intelligent people, which is not neccessarily true at other companies. If you want to, it's possible to learn and grow well, at the lower levels at any rate (entry level to minorly senior levels).
Cons
My guess is the downsides are similar to working at many other large corporate companies. Slow process, very slow climb up the career ladder generally speaking. Pay also grows in a generally slow and steady manner, don't expect sudden jumps in pay and level as is possible in smaller environments. Bureacracy and process abounds, as well as politics, as close as even 1 or 2 levels away from entry-level. Generally speaking, Microsoft isn't the best place to be if you want to be a superstar, or quickly advance in your career, or just plain have a lot of energy. Not if you want to get well-rewarded at any rate.