Pros
Good pay. Most accounts are staffed with good people to work with. Relatively secure long-term employment if you are competent and okay with the cons. Management, production, and benefits are all really pretty good.
Cons
The size and type of the clients means that they're typically locked into platforms and requirements that more or less rule out the use of "new" libraries, technologies, etc, unless the project is specifically intended to be a standalone/showpiece campaign. Similarly, attempts by developers to innovate and/or excel are seldom recognized beyond thanks by immediate project members. Coupled with the fact that the prevailing mentality of developers is that of "this is just a job", the result is an environment where you keep doing the same things the same way for a very long time. I'm not exactly sure what it takes for a developer to advance in his/her career here - I've only seen it happen for a handful of the hundreds of employees.