Pros
If you perform you will be noticed. Top performers receive good bonuses. Good employee development and promotion opportunities. Huge company means you have ability to dictate your path whatever that may be. Diverse working environment Good for entry level graduates. Given a lot of responsibility quickly. Company is actually ethical and interested in helping people - can't be said for all of big pharma. Solid drug pipeline and strong growth of current drugs = stability.
Cons
Good employee development means a lot of internal movement which can make things difficult. Despite being rewarding for high performers, poor or incompetent individuals are rarely fired. This means useless bodies clogging up higher individual contributer positions.If youre lazy I suppose you could call this a pro.... Some systems have gotten too bureaucratic making process improvement difficult at times. Big company means some people you interact with / rely on are not in your reporting structure. This can make it difficult to resolve delays or problems with these people.