This is a build I found to be a good balance of burst, which will not fall off late game, then moving into more durability. Given Kha'Zix and his ultimate, I suggest getting Evolve Spike Racks at level 6, then Evolve Enlarged Claws at 11, and Evolve Wings to finish it off, you can use these suggestions to be a very effective top lane, ADC alternative, or either lane on Twisted Treeline. I expect the order of this build would not be very beneficial for a jungler, although I'm sure with minor tweaks, it could be done.

Reasoning for a Brutalizer early game, is due to the massive amount of damage you can deal with your Q, although late game the Ghostblade will fall, so I just recommend selling the Brutalizer when you need the space for the rest of the build. A good tactic to use with the Guardian Angel, is immediately using your Improved Leap upon rebirth, and if they close in on you, using your Improved Void Spikes to slow their attempt.

Kha'Zix is incredibly effective at going toe-to-toe with almost any champion, his Q is incredibly destructive after being evolved. Waiting for the right moment to Leap on to an unsuspecting jungler and hitting them with your Q, Taste Their Fear, Trinity Force, and Evolved Spike Racks will almost guarantee a secured kill.

Pros:

Extraordinary Assassin
Great Harassment
Late game build makes him an effective brawler/nuke.

Cons:

Throughout the entire game mana will be an issue,
the occassional mana potion can be very beneficial.
Early game he can easily be subdued and killed if you're not careful.
Currently Kha'Zix is thought to be Overpowered,
making him a prime target for focused damage and crowd-control.

Your Q has decently low mana cost, and can be used for securing a minion kill occasionally,
although repeated use can result in vulnerability due to low mana. I generally only suggest using your Spike Racks for emergency health, although it can be great for harassment combined with a few mana potions.

Late game is where Kha'Zix will really shine, he goes from a 1 v 1 nuke, to an all around bruiser. Utilizing Evolved Spike Racks and Taste Their Fear will give your team a distinct advantage in team fights.