The main benefit of working at BP is the camaraderie amongst colleagues. I worked in a subsurface technical discipline in the UK, and the colleagues are not only technically skilled but (usually!) excellent colleagues too.
The downside - the corporate overheads, as expected of any company this size. I had nothing but poor interactions with HR when it came to job advancement or changing roles, particularly in the past few years where everything has gone through the internal job board. Talented people are regularly overlooked for roles that would benefit both them and the company and it has become more about ability to BS and network.
I would work for BP again, but HR/corporate has a lot to improve on.
Free lunch - in the UK!, good compensation
Management / corporate culture