Pros
They make solid offers to get you on board.
There are some really good people working there (like everywhere).
Omnicom benefits package is solid.
The upper level creative directors are great people who truly get it, but they're always traveling so their effects are sporadic.
Cons
Unbelievably boring culture.
They use layoffs as business strategy, so you could be fired because the numbers were off that year. This causes everyone to constantly wonder if they're next.
Whispery atmosphere and poor communication.
The creative process is treated like manufacturing. There is no respect for art and there is an unhealthy obsession with technology as the solution to every creative problem from upper management.
It's a promo agency, almost exclusively, so every project is some sort of sweeps or giveaway, and the clients encourage the agency to dig deeper into every problem they already have.
People tend to leave quickly or stay for a long time.
They promote insanely slowly. 3-4 years between each position, regardless of talent.
Perception and politics matter more than talent.
Nepotism. Lots.