From all the advice i have read on this forum and my personal opinion, Cliffs is not enough. I read only a little from Cliffs but that is because i had already started with Schaums Biology Outline and i could see a clear difference in the level of detail between the two. I can honestly say that without Schaums (and my biology background) i wouldn't have done well. You neeeeeeeedd to know the details as much as possible. Not the extremely tinnyyy details but you should have knowledge about enough specifics as possible. All my questions on the DAT could be answered by knowing content in Schaums.
Good luck!