I've played him a few times, and found it quite effective to get CDR items with health. (so you can spam your Q more effectively and still be somewhat tanky). Also, you're going to want to get at least a little AP, to increase the damage of your barrels. Sorry i'm not very helpful, as I only have a few games with him. To answer your question, yes I think he is a viable champion if used correctly. He has a harass ability, free mana/toughness ability, initiate/escape ability, and an ult that can initiate a teamfight, or save a teammate/yourself.