Pros
I have been working in startups for the past 5-6 years and each place had some glaring issues. Either certain teams wielded too much influence or the decisions made by the executives were not to the benefit of the whole team. At A9 I feel like my work is appreciated and I am surrounded by some of the smartest people I have ever worked with. The team really focuses on a work/life balance and making sure you are not burnt out. It's not to say you don't work hard but there are reasonable expectations set. What is really exciting is that you have the backing of Amazon but are able to work in a nimble and fast way similar to a startup.
Cons
The Amazon culture is not for everyone so make sure you know what you are getting yourself into before joining. Certain people will thrive here and others will not like it. There is always a wish to have more perks (free food, sponsored lunch/happy hours) but it doesn't deter from the experience.