Pros
Range of opportunities: Microsoft builds just about every kind of software imaginable so you can switch every few years and work on a different product with different challenges. Smart people: The vast majority of Microsoft employees are smart, nice people. If you've worked at companies where that's not the case, you know how important this is. As an example, I hang out with my co-workers after work -- not because I have to or because I don't know anyone else, but because they're great people that I enjoy hanging out with. Reach of products: There are very few other companies where you can work on a product that will reach millions of users and potentially change the way people do work and live their life. Great benefits: The medical and dental plan at Microsoft are very comprehensive. This is especially valuable if you have a family. Stability: No worrying about whether or not Microsoft will make payroll.
Cons
Stock price: Microsoft stock doesn't increase in price very much any more so getting rich by working at Microsoft is a thing of the past. You'll earn a comfortable, stable living, but probably won't get rich. Big company: It's a big company and has many of the problems a big company has. Sometimes you feel like a small cog in the system. Sometimes there's some corporate beauracracy to deal with. Sometimes it feels like not everyone in the company is moving in the same direction. Frequent re-orgs: These are a fact of life at Microsoft. Most of time, they don't affect the people at the bottom, but re-orgs at all levels happen frequently.