EPO is a hormone that stimulates the production of red blood cells, as well as speeding up the maturation process of new erythrocytes (RBC's). Normally the body releases it when it senses a need for more oxygen, like going up to a higher altitude for example.
Your blood is normally 40-48% RBC's, and epo can boost that easily into the mid 50's. Essentially your blood becomes thicker and your heart has to work harder to pump it around whether you are exercising or not... which is why you get some guys who just go into cardiac arrest while on the drug.
Unless you are a pretty dope athlete it probably won't make a huge difference... and you can boost your crit to the same level using test and eq if your not getting tested. If you're gonna use it, get blood work done every other week at least.