Ventajas
Microsoft fosters an environment of career advancement and personal growth. Movement between divisions (products, HR, marketing, etc.) is encouraged as is training. I started as a tester and have gained the skills (thanks to their training and "stretch assigments" to move into a marketing role. The health care benefits are extraordinary: No copay for doctor visits or prescriptions; coverage for many "alternative" or "complementary" care types (e.g., accupuncture); and substantial mental health coverage.
Desventajas
The annual review process is tough but I do believe it's necessary. Throughout the year you make commitments about the work you will complete. Then you are evaluated against those commitments. Sometimes it's tough because of switching priorities but it has always been fair. You are always expected to go above and beyond those comittments. But I think that's part of what make Microsoft great and its people greater.