I think that having and maintaining your PMP certificate is very valuable to your career as a project manager. The main reason I say this is because to maintain your PMP certificate, you have to earn your PDU’s. Earnig your PDU’s keeps you up to date in all areas of project management. Being up to date in all areas of project management is what distinguises you as a professional from just a practitioner.
Many people question the value of the PMP certificate, because getting your PMP in itself, does not make you a professional project manager. Which certificate makes you a professional? None! Only when you continue to develop yourself and get experience in your field, you become a professional. The PMP certificate combined with the PDU system does provide a framework for becoming a Project Management Professional.
Also keep in mind that certification is valued different in different countries. I have worked in a Dutch country where post-graduate certification in general was not recognized, so there was no return on investment for me to get and maintain my PMP. However, I have been a member of PMI since 1996, and use the PMBok guide on a regular basis. All the project management courses I teach are aligned with the PMBok. I also help project managers prepare themselves for the PMP exam. That is the way I am kept up to date on what is going on with the PMP certification.