Been using ERPPREP for C_BYD15_1908 with great success.
Did pass C_BYD15_1908 exam last week and was a bit tricky: I guess SAP did a new certification exam with a lot of question on SAP Business ByDesign Financials, SAP Business ByDesign Implementation Fundamentals... Much more hard, even with ERPPREP.
Maybe a little update is necessary.
Still worth the value!