Kennen's oriented about managing stacks and doing lots of damage, combined with some great stuns. He's a more mobile Annie; but does less damage usually, and has more range/utility or at least equal utility; both have stuns, but Annie's one is instant and doesn't require the three stacks, not to mention it can be instant and AoE while giving you summon to block skillshots and yadayadayada that's Annie's pros not Kennen's.
If Kennen gets ahead, hello lane bullying. You have to be super careful because he can stun you incredibly quickly and burst you down; against melee's he has a lot of poke which you can take advantage of. His w aa proc I BELIEVE does extra damage with AD, but could be wrong; so you usually start Doran's Blade. If Kennen gets behind, he farms safely with q, waveclears if there's a safe opportunity, can be virtually ungankable and roams. Fun times.
He can be played top or mid, and is usually always a consistent and viable pick. Worth the investment, go crazy.