Ventajas
Big company can support a good benefits package, with formal training and advancement programs. Has a lot of locations and divisions and can offer some job flexibility if one location falls on hard times or if you want to change jobs for another reason. There's even some interesting work if you're lucky enough to find it and keep it.
Desventajas
The worst thing is that it is generally expected that you will frequently work long hours (60 or more) without compensation: no extra pay or time off, nothing. A big company often means frustration in getting things done, always submitting forms or dealing with company bureaucracy. Like all defense companies, not a lot of long-term job security and many jobs don't translate well to other industries.