I haven't used EPO, but I know a lot about the professional cycling world. EPO increases a persons VO2 Max. From somebody I know who has taken EPO, I was told it allows you to push yourself a little more comfortably, beyond your normal lactic acid threshold. People who respond best to EPO are those with lower levels of red blood cells in terms of number of cells and size. Referred to as Hematocrit. If you're an athlete and you have a naturally lower hematocrit level than another athlete who has a naturally higher level performing the same sport, the benefit you will get from EPO is much greater than the benefit another person would get. This is why doctors involved in drugging up athletes like professional cyclists don't give EPO to all members of a particular team. They typically only administer EPO to the ones with the lower hematocrit levels, as percentage wise, they see the biggest improvements from EPO, and in the professional cycling world, they are the key cyclists in the peloton. Lance Armstrong likely had low EPO levels, as once he started getting the EPO shots from Doctor Ferrari, his cycling accomplishments improved dramatically. Somebody who had naturally higher EPO levels would not of responded as well as Lance did with his naturally lower levels.