Here is a message I wrote in another post about Kennen, hope it helps you.
People saying Kennen is underpowered or weak damage-wise probably never really played Kennen. Kennen is one of the strongest mage in the game, at level 3 his R (ult) can LITTLERALLY DESTROY the ennemy team since it deals INSANE amount of damage and stuns EVERYONE getting hit 3 times by any of Ken's ability (Kennen ult alone hits ennemies 3 times). Since he USE ENERGY and if u are being careful, he don't need to go back often in laning phase, his E is one of the BEST ESCAPE in the whole game giving him a HUGE BOOST SPEED for like 2 sec and his poke (Q) has HUGE RANGE, MORE THAN DECENT DAMAGE and LOW COOLDOWN. If u use ZHONYA HOURGLASS u can go invincible while u ult and deal insane damage, and then run out of fight with your E. Trust me, the day you'll face a good Kennen, your team WILL FELL IT HARD. Kennen has ONE MAJOR DOWNSIDE, he takes A LOT OF SKILL to handle properly because you have to DIVE IN THE MIDDLE OF ENNEMIES team to ult and use his W. I would TOTALLY RECOMMEND KENNE TO AN EXPERIENCED PLAYER who knows almost every champion's attack, but I WOULD HESITATE recommend him IF YOUA RE NEW OR INEXPERIENCED because you might be FREE FOOD to the other team. Trust me KENNEN IS A BEASSSST!
P.S.: He is also one of the funniest champion to play, have amazing ''dance'' and laugh, and makes some of the funniest sound in the game.
P.S2.: He is super small and insanely cute and becomes a tiny lightning ball when u press E XD
Right now he is my main, and I haven't lost a single match with him the last 10 games I picked him.