Ventajas
Pay is good. Benefits are decent. Usually not asked to work insane hours. Lots of perks around the office and at offsite get-togethers. Easy going atmosphere. Managers aren't running around yelling and cursing at people. Produce high quality work. Get to work with major industry leading clients and often cutting edge projects.
Desventajas
A little disorganized at times. Some drama kings/queens who act like life/work is always in crisis. Creative folks can be a bit egotistical and emotional. Enormous focus on selling, selling, selling more work (part of regular agency life I guess). A little too big for their britches, in that many people internally are slow to realize that the rest of the industry is fast catching up to them in competency and quality.