Pros
First if all, as with most big companies, your experience at Amazon will vary greatly with your business unit, and your team. Mine are great. I work in the Kindle org, and overall it's a great place to work. The work is definitely interesting, I've worked in backend systems where the challenge is reliability and dealing with big data. The work-life balance is great, and I almost never spend more than 9 hours at work. When I do it's almost always because I'm working on something that's really interesting to me and I just want to get more done that day. I really buy in to the whole customer centric thing. We really do make choices that we thing would be better for the user (than the alternatives) even though it may be more painful for us in the short run.
Cons
The requirements still change very often and you'll find your sprints often interrupted with something that just HAS to happen. The company of so huge that there is often overlap between what different teams are doing. It can help to just get things done quickly, but often it seems like a waste when one of the solutions is eventually discarded.
