"Relationships are the most important thing to understand in PowerPivot". It's one of the first things I teach my students in every single PowerPivot training. Missing or incorrect relationships will lead to incorrect DAX calculations and wrong results in your pivot tables. The funny thing in V1 was that you needed to create and debug relationships in a tiny message box with only 4 drop down boxes.
The good news in V2 is that PowerPivot ships with a great diagram view.