Pros
The best reasons to work at Microsoft are that you get to work with a group of exceptionally talented and motivated people, you have almost limitless career growth, and you will always be working with the latest and greatest technology that is available. Senior management is very competent and will give you feedback on a regular basis that you can use for personal as well as professional development. The people you meet in the hallway usually seem happy to be there (minus the guys that are late to their meetings and coding on their laptops while trying to hit the right elevator buttons) and you can almost always strike up a good conversation with someone interesting at lunch (we're not all antisocial nerds, well at least not all the time =P) If you are a fresher like myself, I have found the environment to be very nurturing yet at the same time there is plenty of challenge so you will feel proud of your work when you are done. At my previous company when management said I did a good job on a project I just shrugged it off, but when someone says you're doing a good job at Microsoft it is really something to be proud of!!
Cons
The downsides are that sometimes you will put in long hours if you are close to release in the software development lifecycle. Occasionally I hear about people having incompetent middle manager/leads which I believe is partly due to the "if you can code, you can manage" philosophy. If this happens be diplomatic about it, give feedback to your leads and senior managers and remember that sometimes they are still learning too. Also, as a downside to having high career growth you can be put in a situation where they will give you more responsibility than you asked for. This is great for experience but it can be stressful if you are not used to it. The way I see it, that is just part of the training and what is life without a little challenge anyway?