Ventajas
The salary is fantastic! I always felt like they rewarded me well for my hard work and extra effort in terms of bonus and stock award. Very generous with vacation, with option to purchase an extra week each year. You will do very well if pay and benefits are your highest priority. The re-orgs can be a little bit intimidating, but if you do your job well, you will be safe.
Desventajas
Notorious for HR overreach; important personnel decisions that should be left to management end up in the hands of young and inexperienced HR staff. There is a problematic tendency to over-staff teams during good years (I would estimate 10:1), then go through protracted re-organizations when the price of oil falls. That said, Shell is a big company, and "upstream" is not representative of the whole. From what I could tell, many of the core-business upstream and most of the downstream folks all seemed to really love their jobs.