From what I have seen, there are three levels of people that work on PLCs.
First, you have the Integrator. They take a customer specification and turn it into reality. This person needs to know everything since they will be designing the system, building the panels, programming the PLC, and debugging the final project. They also end up travelling to customer sites a lot. So if you like a challenge and travelling, this is for you.
Second, is the high end user. This person will optimize existing machines and build their own when needed. Although the machines they build will usually be simpler then the large integrator built machines. This is where I consider myself. I spend my days trying to squeeze out every bit of efficiency out of a machine while making it more reliable, safer, and easier for the operator to run. The negative is that you are always producing charts and reports to explain what you are doing, and why you need money, to people that know very little about programming or machines in general.
Third, is the debugger. This person logs on to the PLC to try to figure out why the machine is not doing what it used to do. They may make small changes to smooth out the rough areas in the program, but that is all. Most of the time they are just blindly forcing on outputs and toggling bits until they either hurt someone, break something, or "fix" tha machine. You don't want to be this guy.
Well, that is a start....