Butterfingers is easy. Carrying with Kennen is also easy.

Pros:

Stun lock entire enemy team.

Deals huge damage.

MASSIVE power increase at level 6.

Short CD on Q, so it's quite spamable.

Uses energy, so you can bait the mana out of your opponent and out-sustain.

Has a great engage with his ult.

Can 1v2 fairly easily anytime after level 6.

Cons:

Not too mobile. Can speed up, but only for a short time to engage once, or disengage once.

Easily baited. If your team doesn't decide to follow up, you have no escape.

Stun is a little bit slow to get off if you don't constantly micromanage your W stacks.

Can't do much against a fed tank.

The main thing that sets Kennen apart from other champs is his ult. Many champs have game changing ults if done correctly, but none of the same magnitude as Kennen. Even if you do the worlds worst ult, and just stun a single player, no matter who they are, they're going to die. You hardly have to try in order to stun at least two people, and then these two are basically already dead from the gross amount of damage your ult does, and your teammates can easily finish the job. Now, once you get good at Kennen, you'll start to get super easy kills from roaming botlane as soon as you hit 6. You'll be a total beast by 10 minutes, and you can snowball all over the map picking up easy kills that you hardly had to try for.

Okay so now you're fairly comfortable playing Kennen. Diving into the backline is hella easy because of your E, and then you've taken out their squishy carries with a quick RW combo. Wow, nice job, that was SUPER tough wasn't it? You'll get at least a kill, probably two, sometimes even 3 in the span of about 3 seconds where their team runs away because half the power in your ult is just the scare factor. There's literally no counterplay. Even if you die, their entire team got stunned for a solid two seconds, and you took at least one or two other guys with you, so it's incredibly easy for your team to clean up.

I'm going to continue making this guide more comprehensive, so stay tuned for more pro Kennen tips.