Pros
I learned a lot. Amazon employs some very smart people who've done good work solving some very hard problems. If you want to know about scaling, look no further. There's no question that having Amazon on your CV is good for your career.
Kontras
Poor direction from senior management who couldn't decide what they wanted. Bureaucracy. Oh, the bureaucracy. Politics. Woe betide you if you worked in a group or on a project that senior management didn't find sexy. Churn. Despite intensive recruiting, people left faster than they could be replaced. The hiring bar was rightly high, but there were too many ways to fail the interview for spurious and/or arbitrary reasons. The recruitment process may work better in Seattle where Amazon is one of the biggest names in town, but the competition for developers in London is too intense. There are other prestige tech giants, there are big media companies, there are startups, there are banks with deep pockets. In this environment, Amazon can't turn down good people and meet its hiring needs (and indeed, they had to import lots of staff from Seattle to keep the office afloat). On-call. Being woken up in the middle of the night got very old after a while. And finally, the kicker: work-life balance. The company pretty much destroyed a few of my colleagues. Amazon are very good at using guilt to get people to work all the hours God sends, for no particular reward. At the end of one spectacularly hellish project, everyone got -- a T-shirt. Not the best way to make your employees feel valued.