I currently have Bjorn as my new favorite character, and have won 200 games with him. He is not underpowered in the slightest IMO.
His stun is very slow, but other than that I really like everything about him. He is rather tanky, has an escape, damage over time and is medium fast at taking down turrets.
My only advice would be to run him solo lane, as he loses to most supports.

Edit: If he were to get a buff I would be happy, but I don't think he needs one as he is right now, he is not the best but he can compete.