Pros
Might get a decent post that's close to home and works with your schedule (if you're lucky). I was fortunate enough to work Monday - Friday dayshift with weekends and holidays off (client was closed holidays and didn't require coverage). Field management, while being constantly short-staffed and lacking sufficient quality guards, were generally fair. A few had a tendency to micromanage and didn't care if you had other guards that would be late or no-show.
Cons
They will not pay you out properly when you leave G4S; took them 15 days to pay me out. At least as CA labor law is concerned, if you give at least 72 hours notice they have to pay you out (pay, PTO buyout) on the day you leave. At-will employment works both ways. No advancement unless you're former military or retire LEO.