As an aside, I think there was a study on caffeine where they put people with varying levels of caffeine tolerance through a series of performance tests with and without caffeine, and it showed that even people who had built up a high tolerance and didn't consciously "feel" the effects of caffeine still experienced the same performance benefits of the caffeine. Sorry I don't have the details or the reference (so I can't speak to the robustness of the study), but it's out there somewhere.