I have mixed feeling about these two approaches to marketing. On the one hand CPM is usually cheaper then PPC at most advertising venues. I use CPM to refine an add and test landing pages, because of it is cheape then paying for clicks that may not convert well. When I find the right combination of words or the right banner, I'll sometimes convert the campaing to PPC. However, I have had excellent results also with CPM. Some tell me CPM is for branding ...