Pros
bp wants to change. It's got the desire but just struggles with taking action. Working at this company you have the potential to make an outsize impact on the environment, if you could just get something to stick... which is the hard part. Great benefits.
Cons
bp is an old company with old ways of doing things and it doesn't know how to change, both in terms of working practices, but more importantly in terms of sustainability. Strategy is constantly farmed out to McKinsey consultants who've barely been working a year. Senior management is constantly in flux meaning very little stability for any grass-roots ideas to take hold (where stakeholder support is key). Deep hierarchy means many won't stick their neck out and say what they are thinking - either through apathy or fear.