Pros
Big company with lots of divisions. If you can figure out how to navigate it, offers lots of opportunities. Great benefits. Fantastic health care. Many of the policies are not well defined, so one can use them to their advantage if he/she so desires. Almost never fire people unless something is discovered in their past during background checks. Otherwise stable employment. Power industry is also forcasted to grow over the next few years, so industry as a whole is stable. Good job security. Reasonable vacation and liberal with sick time. (again this plays into the lack of formal policies). Good resume builder.
Cons
Big company. Overloaded structure and lack of flexibility to quickly respond. Not a good use of corporate strategies or sharing of inter departmental successes.