Pros
GP has a good work atmosphere (besides compensation, see cons) and does well when it comes to sharing knowledge and resources. You can usually find help from an internal source so no one is left on an island. There are opportunities to change jobs, more laterally than vertically, but that is probably the same for most large corporations.
Cons
The compensation process is a nightmare. Managers can put you in for a raise, but they have no say in the final answer. Senior managers make the decisions, but they have no idea what value you bring to the company. In the end, you are left with no raise in compensation even if you are below "Market Value". The only way to get a raise is to get an offer from another company, then GP will match it immediately. Why didn't they pay you that in the first place? Definitely not Market Based Management.