I have a PMP... However, just like a college degree or even an MBA it doesn't separate the high performers from those who just got by. I have seen PMP's who were great leaders and managers, I have seen PMP's who could not manage their way out of an elevator... and I have seen great leaders and managers without PMP certifications.
Having a PMP certification does not prove you are a good manager or a bad manager... it proves that you understand the concepts of effective project management (not necessarily that you can execute).
However, the PMI concepts are very valuable. They give you insight into things that you may never think about when you run "projects." Ensuring communication within the team and to stakeholders external to the team, managing and tracking risks, ensuring that everyone understands the scope of the project, developing and managing schedules and budgets, etc.
PMI concepts are not recipes or step by step guides. They provide you with concepts to think about and take into account when you do work. Be careful with people who run "PMP" like it's "gospel" or that you have to do an MS Project Schedule... that is NOT what PMI says to do.
Also make sure to remember, as much as you plan... life will never goes as plan, so remember to revisit and revise your plan often... and that you can't plan for everything (something many PMP's don't understand or believe in)