Avantages
It's a good way to enter the industry. Unlike many employers they are (or were before the current crash in oil prices?) willing to hire younger employees and invest substantially in their training. They offer many travel opportunities -- one even hears rare stories of travel to Paris (as opposed to the more common location in -- take your pick -- ND, OK, TX, etc.). Most of my fellow employees are intelligent and friendly, although it's such a large organization it's impossible to know if this is uniformly the case. And hey -- the company stock has lost roughly half its value in the past three months and they haven't fired anyone yet, so it certainly could be a lot worse.
Inconvénients
The organization is very, very large. It's important to emphasize that -- as a relatively junior employee, it's easy to get lost. The place can be quite bureaucratic and is full of rules, regulations etc. -- keeping up with all the paperwork can be draining, especially when you're still expected to keep up with an ample workload. And about that workload -- it can be intense, especially for field personnel. The pay really isn't sufficient to cover the hours I put in, although there are many jobs within the organization which are better paid than mine. And finally, as a critique of the industry more than a problem with Schlumberger per se, at the end of the day you're little more than a technician. There are only so many iterations of the same problems, a pretty constant set of solutions and devices which Schlumberger sells to solve them, and once you've seen them all (or even just most of them) it gets pretty boring and pretty routine. Creative thinkers would be better served somewhere else.