Avantages
Amazon has a lot of interesting projects going on at any given time. You will likely be dealing with projects that have a scale that few other companies can compete with. This will well prepare you for your next job. If you're interested in a project that you are not working on you can still access the code and explore the development environments. This helps when starting a new project so that you can see how others have done things before. Amazon tries to hire smart people, this means that you will likely be surrounded by people that will be able to teach you something.
Inconvénients
The biggest downside is also one of the upsides. There are so many projects being actively developed at one time that often the wheel is re-invented many times. This can makes it difficult to decide the right way to do something. There are also a ton of proprietary tools. The build system is very complex and some would say overkill for java development. That's understandable considering it was built for complex C++ linking and library managment. The deployment tool is very robust, and thus hard to understand resulting in a large learning curve for even the more senior developers. Hopefully someone on your team knows how to work the thing!