Pros
G4S is one of the largest security firms in the world, and in the USA. They have numerous different post you may work at, and transferring from one to another is a definite possibility. It seems fairly easy to get hired as a security officer. The pay depends on the post, but G4S often pays a little better than other security providers. The work is usually not difficult. It's a good job for retirees and for students generally, but it all depends on the post and the hours you work, not to mention your supervisor.
Cons
It's a job and not a career. You will get paid anywhere from $8.50 to $12 an hour. If you are a supervisor you will make a little more than the others, of course. The benefits are weak, but they do have something available for health, life insurance, and vacation. When you start they will most likely tell you the post you will work, the pay, as well as your hours. I was wide open when I started, so it could be different for you, but I'm sure that they prefer people with good availability. If you have great experience, but a restricted schedule it may prevent you from being hired. They have a lot of turnover with their security officers.