I decided to make this guide, or better just small recipe how to build Kha'Zix, who is in my opinion one of best AD burst champion in game. My build is focused on huge damage dealt by skills and few auto attacks in short time and then run away. So I'll not use stuff for Attack Speed and similarly.
PS: This guide isn't dogma. You should always adapt your game style and item build to current game.
PS2: Please excuse my English, it's not my native language.

Pretty standard AD runes. They are best in my opinion, but you can change them for runes that you prefer (for example: Greater Mark of Desolation, Greater Seal of Evasion, Greater Glyph of Cooldown Reduction, Greater Glyph of Scaling Magic Resist, Greater Quintessence of Desolation, Greater Quintessence of Movement Speed...)

Ignite - Great spell, help you finish running enemy, reduce enemy's healing or at least revenge, when you are dead.

Flash - Another great SS. With it you can catch enemy, initiate or escape even through wall.

There are another good Summoner Spells as well, but these two are best so why change them?

Skill sequence is optimized for quick damage dealing via Taste Their Fear and retreat to safe with Leap. You can use Leap for initiating as well, but you have to count with quite long CD on it and no other escape mechanism (except Flash).

About evolving skills:

• Taste Their Fear - Evolve first. It gives you longer range (for ability and basic attack) and greater damage to isolated enemies.
• Void Spike - Evolve last. Nice splash damage, nice heal and great slow (if you have active Unseen Threat.
• Leap - Evolve second. Longer range and chance to instant cooldown reduction.
• Void Assault - Simply, I don't think evolving this skill worth it.

• Unseen Threat (passive) - Great passive. Deals nice damage, slows enemy, works only against enemy champion (so you don't waste it by miss click on creep) and scalds with evolved Void Spike, so you have ranged slow. Additionally, it reloads every time, you are not visible to enemy, so you only need to go into bush, hit enemy with passive, go into bush, hit him again, etc.
• Taste Their Fear (Q) - Main damage dealing tool. Low CD, low mana cost and great damage output. And if is enemy alone (isolated), so there starts funny :D
• Void Spike (W) - Kha'Zix' long range weapon. Well not exactly LONG, but at least something. Deals some splash damage (usable for last hits) and heals you if you are in explode radius. But when you evolve it, there is totally new spell. You can wipe whole creep line, heal yourself and slow enemy. What else to say?
• Leap (E) - You can jump into teamfight, deal some damage and jump away. Great spell. If evolved, Leap has really long range so you can jump over thick walls easy.
• Void Assault (R) - Great passive (every rank gives you evolve point which is Kha'Zix' bread and butter). Additionally gives you two one-second stealth in 10 seconds which helps you reload Unseen Threat, surprise enemy or run away. Evolved version gives you three stealths and damage reduction, which is due to another evolved spells useless.

• - Cooldown Reduction, there is nothing to discuss.
• - CDR, damage and armor penetration. Great item for any AD bruiser eve if you don't build Youmuu's Ghostblade.
• - Every single part of this item is usable for Kha'Zix. HP, mana, Attack Damage, Movement Speed, Slow and mainly passive from Sheen - it deals terrible terrible damage in cooperation with Unseen Threat.
• - Very important Life Steal and scalding AD.
• - Useful AD and Magic Resist for sustain + nice passive.
• - Necessary sustain and great passive, because Kha'Zix is quite suicider.

Some meta items, which can be used in order to current game and enemies:

• - Take if there is too much AP casters or CC in enemy team.
• - Take if you are bothered by AD carries in enemy team.
• - If you build Kha'Zix tanky (with some HP items such as Frozen Mallet or Warmog's Armor), take this.
• - If you need some sustain and protection from enemy casters.
• - More sustain than Trinity Force and sure slow.
• - Interesting item which will heal you as hell thanks to Spell Vamp.
• - Need more damage? There is an answer.
• - Enemy tank has too much Armor? Just ignore it.
• - You're getting raped by enemy AD carry? Just send them their damage back.
• - If you need quite more HP, take this.
• - If you don't know, what to do with money and your The Brutalizer, upgrade it into this.

If anyone wants to know: there is chance to get all (4) spells evolved. And there are two ways how to get it:

1. If you play against Rengar, at 16th level if you have 3 spells evolved and Rengar has Bonetooth Necklace, quest called The Hunt is On! trigger. You have to defeat Rengar (your team can help) and then you'll get 4th evolution point. On the other hand, Rengar will have same quest, so if he kill you, he'll get item called Head of Kha'zix which is Bonetooth Necklace with all stacks.
2. Second way is to achieve penta kill.

There is so many ways how to play Kha'Zix and this is mine. I hope you can use some stuff from this guide and I am looking forward to some feedback.