The PMP certification is mostly useful for those project managers who don't have a solid experience in project management. It will help them get jobs that they wouldn't be able to get with their weak experience in project management.
The PMP market is getting hotter and hotter because there are so many would-be project managers, driven to the profession by tales of $100k+ salaries.
As for usefulness in terms other than career advancement, I think the PMBOK is a good on project management that teaches new project managers how project management works and gives them the basics that they need to handle this profession.