I consider ERPPREP as the best source for preparing certification exams. Specially for those who have domain knowledge but can't allocate their entire time in studying because of a regular job.
I bought the service for Activate Project Manager exam and praticed the questions for numerous times (Both Full and Mini Exams). It has given me lot of confidence and helped me to score 77% in the final exam. The questions on ERPPREP are very insightful and provides an overall exposure to the topics which are covered in the actual exam. You can always refer the SAP course content in case of any doubts on the question asked on ERPPREP. Which would help you to gain more knowledge.