Avantages
It can be very intellectually stimulating - I work on signal processing algorithms and its fun seeing how the 'real world' works vs. all the theory from school. They paid for my masters degree, I've had a chance to work on some interesting programs and travel to interesting destinations. It's also nice working with other smart people - something that is often overlooked in Aerospace and Defense. My bosses have been very flexible with work/life balance (although that is not universally the case). I also feel I made a lot of money out of school and continue to do well vs. my peer group (both in and out of defense).
Inconvénients
Raises become much smaller as you accrue more service time (mine was basically the same as inflation was last year). The training budget has become much smaller recently so anything beyond the necessary training has the really be supported by upper management. My line management spends very little time getting to know me personally - I really feel like a number a lot of the time. The timeframe on projects I've worked on are typically very long - on the order of years (although that is not the rule). When I told my manager that I would like to work on different programs he said that he feels comfortable when an employee has spent five years on a program before moving. If they make me work five years I won't be there. I've been searching for some real mentoring but I feel like I have to do everything myself.