Avantages
1. If you love tech, this is a great place. No doubt you'll talk tech (mostly the MSFT stack) from enterprise to consumer - from PCs to phones to Xboxes - from datacenter to desktop.
2. What were GREAT benefits are now VERY GOOD (took a small step down) but still probably better than you'll find at 99% of large corporations. If you've got family - the value of the benefits is even higher. 401k match is nice.
3. Even with it's struggles MSFT is still a cash printing machine. This means if you can keep your nose clean and do reasonable work, you can have a stable job, pay your bills, feed your family, and not worry (too much) about layoffs. The stock you own likely won't tank, but probably won't go up much either. You'll get a bonus each year and some stock. It's a decent life if you aren't looking to light the world on fire.
Inconvénients
Brand on Your Resume: After many years of losing market share and struggling to be at the front end of innovation and the fact that there's 90,000 employees, don't think MSFT is necessarily going to be attractive on your resume to more agile and smaller companies.
Managing Your Career: Make you say this out loud so it registers - 90,000 employees work there. Double that for vendors. It is VERY hard to "stand out" and move up in the company. Don't expect your manager to be much of an advocate or enabler to help you meet your career goals - they are basically trying to survive the stack rank every year too. Not familiar with the stack rank? Check out the 2012 Vanity Fair article called "Microsoft's Lost Decade".