Pros
Great work-life balance. Great bosses. Great colleagues that you can rely on. Private or shared offices (team size/facilities permitting; expect a three-plus year wait or management for privacy in my experience). Good facilities. Pretty good equipment. Lots of cafeterias with varying kinds of food. Company name recognition and respect. Free sodas and juices.
Cons
A not-invented-here mentality. The company is split into fiefdoms that look out for themselves first. The CEO, Steve Ballmer, is a clown with no vision, inspiration, or appeal. No cool perks like at Google: no free food, no massages, no campus bicycles, no 20% time, etc. There's a stink of stagnation and irrelevance. You have to use Windows and Microsoft tools (no UNIX terminal or programs). Huge wastes of time: dependencies are often constructed in parallel, and the stuff you need just isn't there yet (contrast this to an iterative approach). Most tools are home grown and not supported very well. No Git or other decentralized version control; it's like CVS or SVN, but a worse Microsoft version. Terrible Sharepoint wiki software used for internal wikis. Often times, some internal tool doesn't work, and you have no idea who to talk to.