I've played a few games with Yorick.
All I can say is...well....he sucks.
Its not like he's bad, he just doesn't excel in anything. Everything he does, theres another champ that does it better. Sure his minions are a cool idea, but they pretty much die as soon as they get used.
He's extremely slow so its pretty hard to take people down. His damage isn't the best. He is pretty tanky, but even tanks can out do him in damage. And they have more HP.
He needs a few alterations to make him a good champion. Several buffs to let him run with all the other champions.
However, Yorick currently pretty much just drags down the team without much contribution.