Ventajas
You will be working with some VERY intelligent people. Salary and benefit is 75 percentile in industry. Compared to other industry, pay is pretty good. Shell has pretty good work-life culture. The European influenced culture values people and their opinion. You may be exposed to large scale, complex, international projects.
Desventajas
Since 2008 Royal Dutch Shell and Shell Canada have been in constant change. I cannot remember a single year without some kind of re-org, and it seems like it is still so and people are getting tired. It is a great company to work for if you are a well engineer or geologist. All other field are considered "non-technical" and there is limited carrier opportunity unless you move up. You will be working internationally, from your desk.