Pros
Wide variety of resources available to you IF you're creative about it. LOTS of very talented people around, but not always where you expect to find them. I was very happy to work with the group of people that I worked with. They were extremely competent and I learned something new every day.
Cons
As with any large company, there are a lot of mediocre people "hiding out." Some long-tenured project managers sit in their offices like princes, resisting new technology and doing little to contribute to the development of junior staff. With no one else to look up to, inexperienced junior staff tend to idolize these managers despite their shortcomings. This dynamic results in a hectic environment in which people take turns throwing each other under the bus as they scramble about trying to put out fires that could have been avoided through proper staff development and delegation. Cronyism at its finest. Another issue is the oppressive accounting bureaucracy. While it is true that some departments (see above paragraph) do need accountants to monitor them, some of the accounting policies and procedures are cumbersome to such a degree that they are actually self-defeating. During my employment with this company, there was a distasteful feeling that the company was milking its employees for money. For example, it marketed "money saving deals" to us from various companies, and also the opportunity to purchase things through advances on our paychecks. Another example: the 401K was managed through Merrill Lynch but invested in Vanguard funds. Why not eliminate the middleman and reduce fees by going with Vanguard directly? Hard not to suspect that AECOM higher-ups were benefiting in some way from sales made through these avenues. Finally, the degree of propaganda that this company used was downright creepy. It was bad enough that our health insurance got worse; did we need to be sent health care advisories with stock photos of people smiling and laughing as they inform us about the limits of our coverage? Images of attractive, smiling people aren't going to mask the fact that our health isn't the real priority here.