You know it really is a topic that is up in the air, although most pre workout product contain CrM and caffeine. I have seen the studies stating do not mix CrM and caffeine, as you all know, one of the primary ways creatine works is by loading the muscles with water,which why consuming large amounts of water is such a critical part of effective creatine supplementation. On the other hand,caffeine has a diuretic effect,it draws water out of the muscles and the body. So if you take your creatine simultaneous with your caffeine, you’ll tend to cancel out the cell-volumizing effects of the creatine.
Now I don't know quite what amount of caffeine is needed to effect the creatine though.