this was huge with cyclists and runners a few years back, they have since come up with a test for it, so it is now used less but there are other things that are similar being used - as well as the traditional blood doping.
the main problem is that epo increases the red blood cell count - so you have more of them floating through your veins - which means if there is any sort of pooling (like you sit or lie down in one place for too long) then the things bump into each other and the arterial walls - which can then trigger clotting...
then this clot can either block the blood flow - or it can dislodge and then go to other bad places - your heart meaning heart attack, your lungs, or a stroke if in your brain.
I know the american cyclists coach was said to have them wake up every few hours and move around so that the blood wasn't pooling for too long.
I personally would avoid the stuff. some steroids raise the red cell count as well - I think I recall deca being one.