Au Sol isn't played much because he has an extremely high skill floor, regardless of what people like to think. He's an immobile, sustained damage dealing control mage. He's weak to assassins, has very little counterplay potential and just isn't as good of a mid laner considering all of the absolutely broken hard counters he has.
Pre-6 he's essentially a bitch to anyone with a gap closer. I think he's better top lane as a counter to tanks and anyone without an initiation and better as a support with his stun spams and damage output, not to mention crazy peel potential with a Rylais.