You can not grow, either in skill or in your career. Your work is primarily only seen internally, and depending on the kind of work you do you cant use it in your portfolio. 65-70 hour weeks are common and not appreciated or rewarded.
Strong company with good benefits package and job security. There are occasional opportunities to work on a project with reach and creative flexibility. The company tries to provide well for the continuing training and talent development of employees, through tutorial sites, conferences, etc.
Cons
The physical environment is unappealing, you work in a windowless box. The daily project fare is pretty stale and repetitive, as well as standard corporate mandated trainings and staff meetings. The direct management of the creative team functions rather more like administrators than creative directors.