He is viable. He can be played win lane and be useful late game due to all of his CC. He is however probably not IDEAL as he does not have any real AD scaling skill or steroids.
Main thing to keep in mind is that he is meant for laning phase, and you can't just walk into the enemy team and use your ult like you would with AP Kennen. Typically, his ultimate is used to keep bruisers off of him.
Honestly, I think he has no chance of survive vs AP kennen. due the fact he will not benefit on his skills by the AD build. Yes you might have better auto-attack damage and such. But once AP kennen has full build. He can really flip the table once he engage in team-fight. Luckily I was once in team vs AD kennen, all I can say is that kennen was a feeder.
Kennen AD still around, but not as useful/powerful as AP. That's the point I'm getting to.
He is ranged, has a decent and attack animation, an escape/repositioning tool, CC, his stun stacks faster with Runaan's Hurricane and it scales with his own AD. His faults are that his range isn't that brilliant, using Lightning Rush needs practice (since you can't auto-attack during it's duration/it's your only escape). He's not top tier at all, but he works nicely.
Kennen AD still around, but not as useful/powerful as AP. That's the point I'm getting to.