Kennen has been an overlooked champion for a long time. He has always been viable in my opinion and after the buffs this patch he is a top tier champion.

This guide is just to show you how I build him and play him. It is not to show you how to pick the champion. It is not to show you how to type in your password in the login screen. It is just to show you my build/item choices and why I decided on them because I get asked it a lot.

Here are some replays on my smurf of kennen.
http://clgaming.net/replay/2740/
http://clgaming.net/replay/2741/

I use a full ap rune setup. Flat ap quints, magic pen reds and ap per lvl yellows and blues.

I do 9/21/0 kennen can get away with deep defense and it makes him amazingly tanky through out the game.

Also ardor helps out with your end game ap.

I put a point in Q every time I can.
I max w second.
I get one point in e at lvl 4 so that I can escape ganks and close gaps. I max e last.

Of course get R every time its up also.

I run flash ignite.
Being able to flash into clumps of people to initiate is key. Ignite is just for the extra punch.

I like to start doran shield on kennen so that I can stay in the lane until boots+ hex tech.
After hex I will grab will of the ancients and my level 2 boots. The choice of boots depends on the enemy teams amount of cc. If you get mercs then you will get void staff after rylais. If you get sorc boots then you will get abyssal scepter after rylais.

After you have your spell pen item you can grab deathcap and lastly zhonyas hourglass.

Kennen can be extremely aggressive in lane or he can be very passive.
You have to understand the match up in lane before you go into it. Either way try not to be too aggressive until lvl 4. Once you are lvl 4 you have your e to let you escape ganks and you can be much more aggressive.

If you play passive you can focus on last hitting with q and auto attacks. Q is amazing for last hitting so get used to aiming it properly. Remember FARMING IS EVERYTHING. So if you have a choice between harassing and getting some last hits. Go for the last hits. Farm wins games not harass.

To harass charge your W up to 4 stack hit them once then W. Also you can try to hit them with Qs but most good players will dodge them. You have unlimited energy though so spam spam spam!

Once you hit level 6 you can wear the person in your lane down with q and w harass and if the person goes to 50 percent health you can flash ult after charging 4 stack on your w. If you land your Ult+E+Q+W and ignite this will guarantee a kill on almost anyone. So learn to skill shot!

Kennen can be an initiator or a follow up Aoe dps stun bot. Look at your team comp and understand how you work in it. If you have an initiator then go in after him most of the time. If you don't have an initiator then its on you. You will most likely die if you initiate improperly or your team doesn't follow up. So always make sure your team is in position to follow you up. Also let your team know what you are doing, ping or tell them.

Kennen's job is to blow up squishies. So always been on the look out for squishies to initiate on and blow up with your combo.