Pros
Opportunities for advancement and for professional growth: An ever-changing environment can mean great opportunities for the savvy employee who can identify a need that he/she can fulfill. Getting access to decision makers is key to moving up. Market yourself or build relationships with those who have the ear of senior managers. If you are not satisfied with your job, change your role to meet your needs or seek out projects that let your strengths show. Talk to your managers (or who you want to be your manager) about your career goals. If you are a valuable employee, they will get the hint that you are not happy and they will want to make you happy so you will stay.
Cons
Very large company (40,000+) can sometimes make individual employees feel like just one of the masses