Pros
An internship at Microsoft has been a great experience and I would highly recommend it to anyone. The pay and benefits for interns is better than most other internships. Above this, however, the way in which Microsoft structures their internships is very refreshing compared to past internships I've had. Intern projects at Microsoft are incredibly well thought out and are designed so that you can complete them during the summer, make an impact, and not have to learn every intricacy of the product you are working on. As for working for Microsoft in general, everyone seems to be very happy with their jobs and I have yet to see much that would make me want to work anywhere else.
Cons
Not really a negative, but I would say that your experience here, much like at other companies, is very much dependent on your group and the people you work with. There of course have been a few stories of interns who work for groups they simply don't like. For the most part, however, the groups are all very enthusiastic and fun to work with.