Kennen is a champion where he can be great or just fall off. He usually get's all of his damage early game, but if fed still puts out a lot late game. He can harass very well with his Q in lane and do a good amount of damage.

There is no order in the guy but I suggest rushing Hextech revolver and boots. Then build up to deathcap then rylais. Then go get will, by this time your q's should be healing you for 130 per hit. That's why spell vamp on kennen is useful. Then do whatever order you want with the items after.

Pros:
High Damage Early Game.
Can Stun the entire Enemy team.
Good escape ability.

Cons:
Can fall off late game.
Squishy.