Pros
The problems you have the opportunity to solve are large and unavailable at most other companies. It's extremely exciting to see your project talked about all over the tech press and to think about how many billions of times a day your code is being run. At Amazon someone who is right out of college can easily have the opportunity to design a big system from scratch without being told what to do by other more experienced people (although this is a negative if you are that experienced person having to clean up after the newbie). This of course allows you to learn from your own past mistakes since you are allowed to make them. And, after you eventually leave Amazon you'll have that recognizable name on your resume.
Cons
On-call! On-call sucks! It's possible to have a decent on-call experience, on one team in the past I had just that great experience, but most of the time it sucks really bad. The worst of all is being on-call for horrible software that you didn't even write and not getting much opportunity to fix it. The work environment is pretty horrible, if you care about working in an aesthetically pleasing environment then don't work for Amazon (although it might be different once we move offices to South Lake Union in 2010). The benefits aren't as good as a lot of other big companies, if you have a family then you might not want to work for Amazon. Some parts of the company are run by completely incompetent people, you really need to get lucky when you start working for Amazon to get a good group, although it's pretty easy to move around once your manager realizes that you're an engineer that Amazon should try to keep.