Pros
Great pay, great benefits, and decent work/life balance. For a baby boomer, it's a dream company. Also, since it's at the top of the food chain, just about any company will hire you with Shell experience on your resume.
Cons
Extremely inflexible and bureaucratic. Nothing happens quickly in Shell, and the inefficiency is costing them. Very rigid job roles - you are a cog in a machine. People will sometimes get angry (to an unprofessional degree) if you push into their territory, even if it's better for the company. New ideas are not welcome. "Reorganizations" every couple years of departments (and in 2009 the entire company!) keep the efficiency low and lead to a constant anxiety about job security. Worst of all, the Peter Principle is hard at work in Shell. At the lower levels, people are sometimes promoted based on their past technical performance and not on their ability to manage people. All in all, a Millennial's worst nightmare of an employer. Retention rates of new hires are very low.