What Do You Know About Used Car SEM and Pay Per Click?

 

A Velocity dealer whom I respect very much asked me what I knew about SEM and Pay Per Click for used vehicles.  I replied by saying that I really did not know much, but we agreed to put the question here on the blog for others to comment on.  I would appreciate hearing the opinions from anyone, good or bad.  Also, what company/tools have you tried, and with what degree of success?