I heard all kinds of things about how it helps with EV training, but what does it really do? And how does EV training really work?
EV training: when you defeat Starly your Pokémon get 1 speed EV. I think that you already understood, that more EVs means stronger Pokémon. So it´s better to EV train on lower level Pokémon coz you get exp AND FAR MORE EVS.
PokéRus: doubles the amout of EVs obtained but lasts for only a centrain amout of time, see http://pokemondb.net/pokebase/1547/how-long-will-it-take-to-spread-pokerus