Pros
You will be working with some of the best and brightest engineers and scientists in the world. There is an excellent culture of openness, equality, and responsibility that has developed as a result of the executive embarrassments of the early 2000s. Compensation is generally competitive, but raises are poor, and bonuses for non-management are paltry. Monstrous company, lots of positions to grow into for your career, if you can find an open one not actually reserved for someone already in the company (they do try very hard to help laid off employees to find other positions inside the company). Health benefits at one time were considered some of the best in the country, but have been recently reduced to now only comparable to industry competition. Still decent, but not awesome. 401k contribution pretty solid, but again not as great as it used to be.
Cons
Executives have made lots of choices the past few years in the interests of cutting costs (a.k.a. layoffs, site consolidation, benefit reductions). For the defense side of the company, the constant contract cycle can be quite stressful if you get into a situation where your employment depends on the whims of the government customer or the competency of your local leadership, which can often be up-and-comers targeted for leadership positions, and have zero concern for their next opportunity - they don't fight hard or always make choices in the long-term interests of a program. This ship handles like a pig. Change is soooooooooooooooo sloooooooooooooooooooow. IT is pretty far behind the technology curve. Far too much still done in spreadsheets or on paper.