Hello. My Name is Carboknight, you probably won't read this part of the guide, but I am going to fill this up with useless information about the champion you want to know about.

Kha'Zix is a melee assassin who specializes in taking out targets who are alone. This being said he is an outstanding jungler and can get off great ganks at both top and mid lanes (not saying bottom isnt his strong suit, just saying since top and mid typically only have one champion in them, it is acceptionally easy). However Kha'Zix has almost no sustain in the jungle so if you are not skilled with him in the jungle, it'll be like jungling Shaco without boxes. So I'd recommend him to all experienced junglers.

There are two ways I play Kha'Zix. Guide 1 shows you my bruiser Kha'zix, which i build for 80% of my games. Now listen, he is still an assassin no matter what. Just because these items make him a bit tanky is for one reason and one reason only: To get the kill and not get really low and chased to death after the kill. Plus in case you didnt know, enemy champions typically go for the assassins, ad and ap carries first. So these items will still alow you to do sufficient damage and still get kills and allow you to live all the way to the end in big team fights. :]
Guide 2 shows you my assassin Kha'zix. I only play this when the other team has little to no cc and there are at least two squishies on their team that I can farm early game. Well, that and i have at least two tanky characters on my team to absorb the damage.

Guide 1:
Pros:

Can take a lot of damage.
Still put out good damage.
Can stealth.
Awesome 1v1 victories and luls.
Very mobile.
Very easy farm with evolved W early.

Cons:

Not as much burst as assassin build.
Requires lots of team work.
Cannot die alot or youre just useless(duh).
Evolving causes you to sop moving attacking and so on. So dont evolve in team fights!

Guide 2:
Pros:

Very bursty.
Faster jungle speed.
Low CDR so you'll be constanly moving around the map farming or ganking.
Easy to get fed.
Ultimate is almost always up for an easy escape or easy gank.

Cons:

Very squishy.
Mana hungry, makes you reluctant to share blue.
Requires teamwork and communication.
Evolving causes you to sop moving attacking and so on. So dont evolve in team fights!

Very standard rune set really. The only thing you would be questioning is the Quints right? Well see here is the thing. The extra movement speed plus boots at early levels makes it really easy to get a gank off before lvl 4. Plus with your slow inducing passive, youll be running circles around your opponent almost guaranteeing a kill everytime you gank properly.

Smite: Duh.
Flash: Another escape or ganking tool.
Exhaust: Very good for any jungle who doesn't have a slowing ability, however you have youre passive and if you evolve your W that will slow him anyway. Can always refresh the slow by activating your ultimate and auto attacking them again. :]
Ghost: Very good for jungles who are auto attack reliant. You can use it, but I would imagine most of your damage comes from your Q rather than your auto attacks.
Ignite: Never seen anyone roll with Smite Ignite. SO I wouldnt recommend it.

EVERYTHING ELSE IS A WASTE OF TIME. DO NO USE. Thank You :]

First I'm going to talk about all the abilities Kha'Zix has.

Passive: Unseen Threat
This is Kha'zix's only type of cc. Whenever Kha'Zix is not seen by the enemy team, his next basic attack will deal bonus magic damage(scales with lvl and +.5 AP) and slows the opponent.
In Depth Explaination:
This is what makes Kha'zix good at ganking. Going in and out of bushes to refresh this, or activating the ultimate to refresh it. Not to be underestimated. Very useful passive. Not worth going Ap for though.

Q: Taste Their Fear
Deal physical damage to a single target. Damage against isolated targets is significantly increased. Kha'Zix passively marks enemies that are isolated from nearby allies.
◦Evolving claws increases damage to isolated targets even further. Kha'Zix also gains increased range on both Taste Their Fear and his basic attack.
In Depth Explaination:
This is Kha'Zix's Bread and Butter. This is what makes him good on taking on solo top, mid, counterjungling champs. The Damage is significantly better when the target is isolated and you'll know when they are isolated by the marking around them. And to make this ability even better, its slightly larger than your auto attack radius, making it a very good chasing attack or harassing attack. Evolving this ability should always be first or second due to the increased damage and increased range of auto attacks and Q.

The Skill Sequence is different if you want bruiser or assassin. If you want to assassinate youre opponents you want to lvl your Q as quickly as possible and evolve either your wings or you claws(wings for ganking/escaping potential, claws for max damage with your Q plus additional range for harass) 1st. If you want to be a bruiser, max your W first and then evolve it early. Its great for farming, healing, poking, ganking, slowing, escaping, and getting lots of assists in team fights, due to the large radius of the skill shot and the explosions themselves. :]