Pros
At Amazon.com you have flexible work times and the option to work remotely (as long as it is in moderation), and the managers are very supportive. These have been important factors for my performance, since I feel that I am most productive when I work without disruption. In my opinion, the best aspect of working at Amazon.com is the fact that you have immediate impact on customer experience. The projects are usually short (on the order of weeks), iterative and deployments are painless, making it quite easy to innovate and get instant feedback. This also means that you get to work on a variety of projects that stretches you on different levels. Teams are usually small, which fosters team spirit and sense of ownership. You are always judged by your merits. Your opinions are heard, and as long as you can make the case they are most often adopted. Amazon.com, today, is more than an online retailer: it has variety of other services to offer; like Amazon Web Services. This means, there are a large number opportunities to advance your career in a variety of aspects -- without you having to change your company. And, did I mention that you will be working with some of the top engineers in the industry?
Cons
Working at a fast-pace company like Amazon.com has some downsides as well. Documentation is scarce and often times out-dated and people mostly rely on the "tribal-knowledge" accumulated over time to get things done. There are ongoing initiatives that address the problem, but I think it is inherent to the agile software development process, which puts less emphasis on documentation and more on delivering shippable software. However, this does make it challenging to troubleshoot a problem (you end up nagging team members, getting frustrated) and also reinventing the wheel sometimes. Amazon.com is a push-to-web type of software (contrast with "shrink-wrap" software, which you are generally off the hook after the project is down, so quality plays a great role), and there has not been much emphasis put on the quality of software until recently. In other words, quality assurance is nascent at Amazon.com, and resources are scarce. I admit that testing a software at Amazon scale is very tough and it is also not easy to find good test engineers. As a result, developers are sometimes asked to take on some QA-specific tasks, which might be off-putting for some. However, I would also argue that this is an opportunity, because there is enough room to try out new ideas and to tailor the process for individual teams' needs. Frugality is a core principle at Amazon.com: don't expect to have access to free drinks / food (except coffe/tea).