Pros
In my expereince, there was always room for advancement with the company, but it often required relocation to remote places. Also, GP invests a lot into their training and development for their salaried employees, especially engineers. Since theyr'e a private company they're all about investing in their employees, which is a rare thing these days. I always had great senior management who led by example and were extremely fair.
Cons
The downsides to working at GP had to be the location. If you wanted to move up quickly, then you are forced to take positions in rural locations. Also, the MBM Compensation philosophy isn't evenly applied across the company, or even across locations. Middle management often comes from the hourly ranks, and is not properly trained.