Vantaggi
Good training, Level 3 Security Certification, chance for Security Clearance, minimal effort (if you're in to that), chance to obtain CHL while training depending on instructors, steady schedule for full-time employees...and a paycheck. Depending on the "safety" of the site it's good for students looking to have a lot of downtime to study while at work, also good for young retirees or people looking for a second income that isn't too stressful.
Svantaggi
The steady schedule also makes it difficult to get off unless you have part timers with open availability or someone is nice enough to change days off with you. "Safer" sites are usually boring w/ arbitrary busy work that has little significance with higher pay while the more dangerous sites pay and budget less so you're very lucky if it's at least an armed site you could be more safe in. Ironic since they don't offer much in health benefits. Also very little upward mobility and company veterans (5+ years) often laugh sarcastically at the idea of getting pay raises.