Pros
A fantastic opportunity to see the Defense machine at work and get an early view at some really cool technology that public gets to see only decades later (if you're lucky to get on a project like that). The salary is decent compared to your peers, just make sure you negotiate well up front. Benefits (medical, 401, etc) are great but the incentive-based packages are non existent for non-managers (revenue sharing, stock options, bonus).
Cons
It's the nature of the beast, but still a downside - everything moves soooo slowly. But this would be a problem elsewhere as well - Raytheon, Lockheed, etc. You will never ever get to see a full project from acquisition to deployment until you get higher up to be included on the acquisition processes and become so useless that they don't move you to another project and let you coast on one forever. Most projects are miss-managed and developed with the Waterfall method which leads to buggy and delayed products. Forget about voicing your opinion if you haven't served in the military or haven't been there a million years. Speaking of which, the "good ol' boys" attitude of hiring and quickly promoting former military over well trained and seasoned engineers is pissing the heck out the generation of folks who didn't grow up with the draft, didn't serve (and didn't want to) and believed they were in a civilian job not the military.