Welcome to my Kennen guide. This is just a basic guide how I play Kennen. Some parts (like runes and masteries) are the same as some other popular Kennen guides, but that's just because I prefer such build aswell. Other things I do a bit different, like buying order and skill sequence. Feedback and/or votes would be appreciated. Enjoy!

Pretty basic rune choice. Magic penetration, some extra health because Kennen is kinda squishy, AP glyphs and quints.

9 points in Offense for the magic penetration. For the rest I prefer defense to give Kennen a bit more survivability. Make sure you don't choose mana/mana regen things, since Kennen uses energy.

Item choice is pretty basic aswell. The items are in the order that I buy them. After buying the Will of the Ancients I always buy a giants belt first for some extra survivability. After that I go for the deathcap after wich I upgrade the giants belt to a crystal scepter.

An other item I like on Kennen is the Spirit Visage: nice CD reduction, energy regen, some health and some magic res for an acceptable price. ALso a Lich Bane can be usefull in my opinion: it gives your normal attacks an extra boost and the 7% extra movement speed is also a welcome bonus. Screw the fact that you dont need or use mana. ;-)

Untill lvl 6 I get my Shuriken to lvl 3 to haras my enemies as much as possible. After lvl 6 I maximize my Lightning Rush and Electrical Surge first for maximal aoe damage. Especially lightning rush I like maxing as soon as possible, so it has a low CD and you can use it to movehttp://www.mobafire.com/league-of-legends/edit-build?edit=128787# faster. Imo that's really necessary, because Kennen is kinda slow without it.
Other guides max the shuriken first and the lightning rush last, but I feel that the aoe and the increased movement speed is more important then a slightly better shuriken.

Flash and ignite. Nothing fancy. Exhaust can also be an option, but I prefer Ignite for some extra damage output.

Pro's: Kennen has a nice haras in lane with the Shuriken. Also he has quite nice AoE damage which is very usefull in 5x5 groupfights. Another pro is that Kennen has good chasing and escaping potential, thanks to lightning rush.

Con's: Playing Kennen you always get targetted first and you can die in a second if you make a wrong move. So try to let other people engage first and then rush in with lighting rush, electrical surge and your ulti.
Another con is that Kennen is a hard to play hero. Well, at least that is my opinion; I havent mastered him for the full 100% yet. :-)

I think Kennen is a typical champion to play 5x5 matches with, since he mainly is an AoE hero. Personally I wouldn't play Kennen in 3x3 matches; you're better off with a champion who's good at focussing 1 opponent at the time and deal fast and much damage to it. For the rest, Kennen is an excellent and fun champ to play with!

Farming gold is really easy with Kennen. You can easily kill whole groups of minions by rushing in with lightning rush, followed by an electrical surge. Keep in mind that this only works later in the game when you have enough ability power to deal enough damage to kill minions in one time.

Keeping a lane shouldn't be too hard with Kennen. The only time I have some problems with it is when I face two opponents who play kinda agressive. Most of the time you can do mid lane with Kennen against some mage champ like brand or annie. I always try to haras them with Q, followed by W when I actually hit them. When your opponent gets below 50% health I try to jump at them with E, followed by W and Q.
The trick is to apply 3 marks of the storm when you do that, so you stunn your opponent. It is real helpfull to have good timing on your attacks and rush in when your next normal attack will do extra damage and apply an extra mark.