This is the build that I always use with Kennen in 3v3. Idk if it would work in 5v5 but you can try if you would would like. I normally have fairly good games with him. I always have a positive kill-death-ratio with plenty of assists.

The pros of this build are as followes: good dmg output, good early game harrassment, he stuns fairly frequently with high AS, short cool downs, you can spam spells, you use energy instead of that stupid unreliable resource called mana, and BEING KENNEN. :P
The cons include: you are fairly squishy, it may be hard to get kills if enemey stacks magic resist, and i think that is about it. :)

I alwasy use exaust and ignite because of personal prefference. You could also use goast or flash but they are mostly not needed because of lightening rush.

Take a 9/21/0 mastery. The defence masteries give you more duribility. you can also take atk masteries but you may be killed more often.

The runes I normally choose are atk speed marks, health per lvl seals, cool down reduction per lvl glyphs, and atk speed quints. If you like more magic dmg over atk speed you can also use magic penetration marks which work just as well,same with quints. For seals I just use health because you can nvr have too much of it :P. I also used to use ability power per lvl glyphs and they work if you would rather have power over spamming ability.

Start with a Doran's Shield and then buy Boots of Speed as soon as possible. After you gain more money buy the magic penetration boots followed by Malady. Malady is excelent in early game because of the magic resistence debuff and the AS and AP you get. After Malady get Void Staff for more ability power and magic penetration to hit harder. After that get Rageblade and finish off with Deathcap. if you want more AS you can get Stark's instead of Deathcap or you can sell Doran's Shield.

Take one of each at 1,2,and 3. Max thundering shuriken as soon as possible. After thundering shuriken get electrical surge and then lightening rush. Also, take your ult whenever possible.

I have found Kennen is good with champions that have a stun or slow. This includes: Morgana, Sion, Nunu, Lux, etc. This allows Kennen's stun and the other champion's to stop him for a good amount of time.

At the beggining try to get minion kills with thundering shuriken and if possible you can harrass with too. Getting minion kills is very important. If you get gold quick enough, you can pull off some pretty good ganks at 6 if you have the items.

Hopefully halfway into the build now, you should have a good dmg output. With a lightening rush and then a electrical surge you can weaken minions effectivly. This should allow for easy minion kills with thundering shurken and basic atks. Get champion kills when possible and prepare for a win.

If done right, it should be between 19-21 min(just guessing here :P). The build should be almost done, if not complete. Your dmg output should be really good now and you should be able to kill most champs (if not fed by noob teammates). If you can't solo someone all of the way. you should be able to 2v1 or team fight fairly easily.