Ahhh, caffeine and creatine, the most paired combination in todays pre workout. What role do they play on each other? Synergistic? Inhibitory? No effect? SuppVersity compiles research and sheds light on the topic for us.
The article reviews several different studies with different mechanisms being tested and surprisingly enough, all produced different results. The old theory suggests that caffeine has an inhibitory effects on the ergogenic effects that creatine supplies. One study actually showed that the creatine and caffeine group produced less dynamic force of the knee extensors than the creatine alone group.
The other studies showed relatively similar effects however they were ultimately inconclusive.
Read the full article for the in-depth analysis of how the studies were done
The important things to consider when looking at these studies are creatine related. Creatine is one of the few supplements that has a compounding effect over time. To be fair to the researchers, it would be hard to be completely objective when recruiting subjects who have already been taking creatine and in turn it would greatly reduce the population of the study. Further more, none of the studies researched phosphocreatine levels intramuscularly. They measured blood and plasma hoping to make an inference. Ultimately when it comes to training, it doesn't make a whole world of difference. If you get better training sessions out of creatine and pre workout then take them. Better training is ultimately the goal. As always, be objective and look at the research as whole. If you read the article you can see how one study can make a powerful claim and others seem to wash.
The more objective information you understand and gather the more you can effectively pick and choose the supplements that best fit your needs.