Pros
Many smart and competent professionals, especially in some practices. I have worked with some people who are the best in the business. Aon has a good reputation that is a boon when working with prospects. Aon also has a huge bench depth of knowledge so, with a little digging, I can help even the most esoteric needs of a client. I think the Hewitt acquisition gives Aon opportunities to better sell benefits and consulting to traditional customers.
Cons
Too much internal bureaucracy. Aon is getting very AIG-like with GRIP and Client Promise and all sorts of nonsense. Many senior execs have little insurance industry experience so something that is common practice is given a bunch of b-school buzzwords and introduced as such a great new idea (i.e. Client Promise). Compensation increases and bonuses even when there are truly measurable successes are often poor. Senior management seems to be leading for the next quarter. McKinsey cost cutting has gone beyond fat now and is hitting muscle and they seem to be a three trick pony: cut more costs, acquire more businesses (not so bad, IMO), and make up more silly processes with cute b-school names.