Pros
This is a security company that is more reputable than many others and is globally positioned. This creates a draw to more experienced Security/Law Enforcement Professionals which can help to build the company's overall performance and reputation. The company's far reach has enabled many employee benefits within the realm of discount programs, etc. The general reputation that G4S holds will typically encourage a higher standard of work and more quality Officers.
Cons
In the contract security aspect it is very similar to many other contract security companies. Pay and benefits are contract based, which can be highly variable. Contract security does not typically seem to command much respect in the professional world and the support from the main offices, granted that there is some support, is not enough appropriately exude the professionalism of the positions. This leads to lapses in performance and quality. The pay and benefits for the work performed, as with almost all contract work, is fairly minimal. This is, however, seemingly dictated by the industry itself and how the wages/pricing must be undercut tremendously in order to stay competitive with the myriad of small security companies, as well as the more mainstream companies that have built their business upon undercutting contract bids. G4S upper management could do better in their communications and support of the "on the ground" employees, but they definitely give more communication than other security companies I have been involved with. G4S could have better employee benefits.