Pros
Microsoft is a great company the develops the best software in the world. You will never lack for resources to do your job and their are a lot of very bright, dedicated people you will work with. They give you the best computing resources you want - many employees have 2 or 3 computers. Best of all you also get access to all of Microsoft's software. The latest and greatest versions of Office, Windows, SQL, SharePoint, etc. In many groups, you also have a lot of responsibility - kind of like a startup, where you wear lots of hats and much is expected of you. Most of your co-workers are dedicated, smart and really committed to doing the right thing for customers as well.
Cons
Forget about a life outside of work. You will always have more work than you can do and lots of pressure to complete it all. The bureaucracy is at an all time high and the organization moves at a snails pace for most things. Even though you have a lot of responsibility, there are lots of internal politics and cross company issues that make it difficult to make real changes without more effort than you can possibly believe. You would think that the best software company in the world would have great internal systems, but they don't. Lastly, the pay is horrible. Every year, they come out with lame reasons and explanations that pay is really not so bad and that you should be honored to work at Microsoft, but it doesn't do justice to the amount of work most of the employees put in.