Ventajas
Compensation and the retirement benefits for Engineers are pretty spectacular (I am a fan of ESOP funds). The health insurance is pretty blah. I have only ever been on LM insurance and my father's (manager at a different large company) so my reference point is somewhat small but my father's insurance seemed to cover a lot more, the upside on this is LMs are pretty cheap. You work on cool stuff and hours are relatively flexible (This will be specific to management). LM is also very good to Engineers and I haven't known of any layoffs in my 4 years at the company. It's a very professional environment.
Desventajas
You work on cool stuff but the work you contribute to that cool thing isn't always as cool or interesting as the system at large, this is particularly true for early-career employees. Getting promoted in your current group can be difficult if not altogether impossible (unless you get a competing offer and that point why stay wit the company?). For timely promotions, it is necessary to move around quite a bit. Because of those aforementioned difficulties, turnover can be high in certain salary grades (and on certain programs if the budget is tight) and I have seen many talented Engineers leave due to this slow rolling. Other than that the cons are pretty much typical of other Defense behemoths, siloing, cumbersome process, etc.