Coincidently there's another study quoted in this week's RBR newsletter
(http://www.roadbikerider.com/currentissue.htm) touting the benefits of caffeine, with a caveat for those with heart problems. I have been trying those new (to me at least) caffeinated energy drinks like Red Bull etc on the last couple of longer rides. I didn't notice any heart rate spikes and performance (such as it is) may have been enhanced.