Pros
If you're in Aon Risk Solutions and are client facing, you'll have large, high-profile clients. Aon is, after all, one of the largest brokers in the world. The pay is about average for the industry. Work-life balance is reasonable. After you get the hang of things, you'll be able to switch your brain off (if you consider that a pro...which I do not.)
Cons
I've worked at two of the three largest brokerage firms and in management consulting. Aon is shockingly non-collaborative. In my 10+ years, I'm not sure I have found more poorly trained managers. It's a highly political organization, and the in-fighting and bitterness between divisions and teams is palpable. Aon has a reputation for internal "fiefdoms" and it has created a very disjointed organization. Please do not write my review off; I'm not simply trying to flame an old boss or employer. I honestly want to warn others, so you don't make the wrong call and bounce after <1 year. If you value teamwork and want to feel passion for your work, I cannot recommend you join Aon.