Pros
Schlumberger offers reasonable pay and international opportunities. They are almost certainly the most prolific of the oilfield service companies which means that they have a large number of jobs in a variety of fields and locations.
Cons
With so many employees, you are just another number. Management varies from excellent to awful and you have absolutely no recourse for those that are awful. HR are probably the worst part of the company. Either you are ignored or targeted for punishment. With the recent downturn in the Oil & Gas sector, the company is looking to save money wherever possible, this extends to: - cutting training courses - cancelling parties - denying promotions - preventing all travel - laying off employees and firing them where possible.