Alot of people say, "Kha'Zix Sucks!" Well, I'm here to correct that.
I've been playing Kha in ARAMS and ABAMS to attempt to find a decent build against a group of adversaries AND, I think I've found it.

Pros
-easy Harrass
-great Farming
-great Ganker
-great Chaser
-nice Damage Output
against groups
-great Damage Output to Single Targets

Cons
-semi Squishy
-needs a Good/Smart team
-needs a Initiator/Tank
-Semi Item Dependent
-better against single target rather
than groups
-easily CC Raped

Every time you Level your Ultimate you get a chance to evolve one skill I chose my Missiles (w) first. when evolved your W will shoot 2 extra missiles to either side giving you an easy chance to spam/harrass the enemy AP carry. After completely Leveling my W, I next chose to continue leveling my Q yet with the next evolve I choose my E why is that? well, with your E you gain the chance to tower dive then instantly jump to safely and also the extra distance to the jump gives you a extreme chase ability. Lastly, I almost always choose to evolve my R next due to the damage reduction while invisible and the extra invis chance.

The reason I went with mostly Armor Pen runes was the Fact that when you're mid most mids DON'T BUILD ARMOR on their rune pages so this is amost OP in and of its self. nest with glyphs I chose MR per lvl to counter the mid lane even into the late game. Seals are just to give you that extra boost against and ganks.

alot of people question my the use of my early Chalice of Harmony. Reason is Kha'zix can't continue spamming his missiles with out it. Also it gives your jungler and extra boost in his farm/xp.(Team work is everything LoL is it not?) Everything else is to build upon being semi tanky and still giving you a boost to all your AD stats.