Pros
The work can be fulfilling and is stable for the most part. Clear delineation of tasks and responsibilities - no one wears more than one hat and everyone knows what they need to do. Excellent work flow / process. Good work experience for those interested in a career in advertising - you'll get a feel for what it's like to have to meet big deadlines and how to handle them. Department heads for the most part are very knowledgeable and helpful.
Cons
Favoritism very much exists. There's a need to almost constantly watch your back. The hours you have to put in before big presentations and pitches can be exhausting and punishing. During the off-season there can be a scary amount of downtime. Little room for advancement and you might feel like your creativity is being stifled. Upper management can be largely out of touch with most of the staff.