Pros
The best reasons to work here are benefits, group mobility, and job safety. Microsoft by far has the best health care benefits compared to any other company. Switching between groups every 2-3 years is common and even encouraged which means you can change roles and projects on completely different products while still keeping your job. In contrast to the rest of the economy, Microsoft is secure place to work as long as you do your job.
Cons
Microsoft is the wal-mart of technology. Its engineering process is very mediocre which lends itself to buggy and unstable products. What's missing from its process is a key person to help drive the overall vision of each product rather than a bunch of PMs fighting over trivial features. Office 2007 was a big improvement, Vista a huge disappointment. Also considering its size, the company is a behemoth, decision making is very slow and often times people are doing redundant work.