Vantaggi
Lots of career opportunities as well as variety. There are many different positions available at Microsoft and at a variety of geographic locations. Perks are reasonable and are pretty competitive with the rest of the industry. Jobs are pretty safe at MS (assuming that you are competent). As a company, MS is well-run financially so the expectation of layoffs during this recession are quite low. Since MS is involved in a variety of areas, even when one part of the business slows down, other parts do ok, so things do even out. There are some projects (i.e. Azure) that have the potential to be very exciting.
Svantaggi
Not the most innovative company. Very slow-moving in terms of new markets. Given Microsoft's enormous size (almost 100k employees) it's easy to get lost and forgotten by management. Unfortunately, most of the industry looks down at Microsoft employees to the point where it's embarrassing to admit that you work there. Overall, externally, MS has a bad reputation, but as an employer I do think that their reputation is a good one. It's very difficult to start something new and innovative: only certain key employees are allowed to be involved in such things: the rest of us just slog along. Not a lot of self-reflection within the company (i.e. self-criticism).