How difficult is the project management professional exam?

It can be either reasonably easy or very difficult, it's all subjective (but most agree that it's not an easy exam). Note that even if someone is a Project Manager for years, he or she can easily fail the exam if they only rely on their experience. The PMP is about studying, hard. Memorize the PBMOK, try to solve as many questions as possible, and you'll do fine.