Ventajas
Their benefits are really good if you've worked there a long time. The work is ok, and the salary is competitive. Everyone does their own little part. Sometimes you get to do different types of work because they have offshored some jobs(and that never works out because of time zone issues, communication issues, etc). Shell cares more about safety/training/compliance than anything else(especially making money).
Desventajas
It's very bureaucratic. If the benefits are the one of the biggest pros, it doesn't say much about Shell. You'll never feel empowered to do much because you'll be restricted by 20 other departments, half of which have been outsourced. It's not a good place unless you're an engineer. Everything else is getting offshored, outsourced or eliminated.