Pros
You'll learn a lot and its a growing environment. I used to get really frustrated about the focus on frugality and the not-invented-here and build-it-over-buy-it mentality there. However, seeing how the rest of the work does IT, I'm good with the frugal approach and building it and doing work. Most people out there do IT via spending money on products to expand the domain of the IT manager, without any focus on the business, and without any metrics for success -- Amazon is the opposite fo that. Everything needs a cost justification and its difficult to build empires without any competency. The topgrading environment is actually good in some ways because it does eliminate a lot of the people that you have to put up with at other jobs who are totally useless. The bar raising interviews and stack ranking does actually mean that you might be working with a bunch of other decently talented people rather than trying to drag a department along single handed. I usually get bored at a company after 2-3 years, but I stuck with Amazon for nearly 5 years before getting bored with it because it was growing so fast (grew from managing 400 servers to 30,000 servers in 5 years). At some point, though, I'd learned most of the lessons dealing with scale, and that exposed the fact that I simply didn't like working for the company or the upper management (Bezos in particular).
Cons
Frugality get taken to an extreme. It leads to bad decision making if the payoff cannot be measured. If you simply argue that a technology will be able to be leveraged in the future and is forwards-looking it will go nowhere with upper management (although Bezos gets to make bets like that, but the engineer on the ground doesn't get to make that kind of call). Also, the focus on frugality leads to lots of turnover with the employees as they get treated like replaceable cogs. The topgrading environment is also a little abusive. It doesn't allow for someone who is simply doing a necessary job and doing it well to stay in that job, a lot of good people have been chewed up and spit out by Amazon, which increases churn. They would also do well to relax the focus on topgrading without abandoning it completely.