PROS

+ High burst and carry potential.
+ Easy to reposition with E Leap and R Void Assault.
+ Huge slow with evolved W Void Spike.
+ Great mobility
+ Build adaptability can be assassin and bruiser.
+ Easy to clean up fights.

CONS

- Feast or Famine.
- Vulnerable to CC.
- Has no CC (except evolved W Void Spike).
- Bad at teamfights.
- Dependent on passive Unseen Threat for burst.

Flash
It is an essential and can't be replaced for anything else. Kha'Zix needs it to engage or disengage or gap closing to finish of targets, or dodging skillshots etc. Anything else is just not that effective.

Smite
Standard for every jungler. Just as Flash, irreplaceable.

Challenging Smite
Is paired with the Conqueror rune page. Can be used in game when you are about to duel the enemy jungler. Getting yourself an advantage from your Conqueror and Challenging Smite. It is better for a fighter/bruiser playstyle. While Chilling Smite is typically better for ganking and assassinations. So it also comes down to each individual's playstyle.

Chilling Smite
When getting Chilling Smite you wanna get Dark Harvest too (or Electrocute for more early game pressure and one-shots). Can be used to close the distance to low hp targets to finish them off. Or to escape some tight situations. Can also proc Dark Harvest or Electrocute. Usually my go to since if the team need a bruiser i prefer picking one than going for Kha'Zix and making him bruiser.

Domination

Dark Harvest is in my opinion the best option. Electrocute is good too but like for early game. It feels weak in late. It can be used for more early damage and one-shot potential. Dark Harvest on the other hand needs a bit attention in early to get it stacked and dominate mid-late. So kinda like the way you play you get to choose which one to go for.

Sudden Impact for lethality and thus extra damage when using E Leap and R Void Assault.

Eyeball Collection for more AD and scaling. Can be swapped for Ghost Poro in vision elos like Diamond+

Ravenous Hunter to have better sustain while clearing your jungle since Kha'Zix is an early jungle, you will fight early and benefit from it by getting stacks. And solid healing source throughout the game.

Magical Footwear for a bit extra mobility (with the +10 movement speed) and free boots to be able to get core items (by saving gold) that will get you snowballing earlier.

Cosmic Insight feels really good for the ability haste on abilities and summ spells too.

Precision

Conqueror for more like hard matchups for example, Graves. It provides AD and healing. Good to survive bursts and still be able to deal some damage. It goes with a more like bruiser playstyle than the classic assassin one with tankier items and different teamfight approach.

Triumph to survive close early fights and for better sustain in team fights and skirmishes.

Legend: Tenacity to be harder for enemies to lock you down and chain CC you. Easy way to move around team fights and skirmishes. Legend: Alacrity can be an option of course but again, you will be a bruiser so the tenacity would be more impactful. If though the enemies dont have that much CC you can go for the extra attack speed for sure.

Coup de Grace to be able to deal more damage to low target and finish them off.

Usually you want to grab Q Taste Their Fear at level 6 for 1v1s and burst. Then if you go for W Void Spike at 11 you take E Leap at 16 and vice versa.
In most games after getting Q Taste Their Fear at level 6 you start to snowball. If that is the case i suggest to evolve E Leap at level 11 to transition the lead. (If that is not the case and you aren't snowballing it is a must to evolve W Void Spike at level 11). Evolved E Leap offers reposition, mobility, and a reset on its CD at champion takedowns. And then at level 16 evolve W Void Spike for utility in team fights and a bit of healing. Also if the target is Isolated (passive Unseen Threat) hitting them with W Void Spike also slows them by 90% for 2 seconds instead of 60% that slows without the isolation. Evolving R Void Assault is also viable for more moliity and stealth to take down isolated targets and play mind games. Also it's very fun having 3 times stealth with R Void Assault and more stealth from Duskblade of Draktharr every time you kill and enemy. I mean, fun for you not the enemies...

Evolved Q Taste Their Fear get +50 range.

When fighting and you are in need of more damage, get in a nearby (non warded) bush to activate your passive Unseen Threat and deal the extra damage while also applying the slow. Also it is a good way to check if a bush is warded. Without the passive activated you get in a bush and if the passive doesn't grant he buff that means it is warded.

Your Q Taste Their Fear isn't an auto attack so it can't be missed from Jax's E Counter Strike or Teemo's blind Q Blinding Dart.

For Kha'Zix's Double Jump, you need to kill someone or get an assist while you are inside your E Leap animation. If you cast your Q Taste Their Fear while you are jumping over your target and you kill them while inside the jump, spam E to the next place you want to jump and you will perform a double jump.

When in R Void Assault, Kha'Zix ignores unit collision (he can't be body blocked).

Flashing while E Leap cancels your jump. So when needed, first Flash and then E Leap.

Control wards don't reveal you in your R Void Assault

When engaging use your R Void Assault not your E Leap. Save E Leap for disengage or for gap close to finish off a target.

When the enemy jungler reveals himself, and there is no objective on the map to get or any play to happen, you can counter-jungle the opposite quadrant. Or when they die you can sneak steal his buffs and make him really frustrated. Also, you can leave one small Murk wolf or one small Raptor or from the 2 original Krugs the small one to prevent the camp from respawning. This way you mess up his camp sequence and leave him behind as far as gold and exp are concerned. When the enemy jungler goes to that camp he will have to kill the small monster for little exp and gold and wait until the camp respawns again to take full advantage. If you can do this in his red buff quadrant, he will have 2 camps stalled and his buff missing, and he will be quite upset.

Always check for enemies who shove waves into your tower when your laners are missing (dead or recalling). These minions are wasted and why not take advantage of some free gold and exp.

Do not force ganks except if you really need to (for example, placing a rift and getting the gold from the plates). You mainly want to powerfarm but do not think ganks of a way to mess up your clear and disdain them. You maybe get a kill or if not, you will get exp for sure and some gold that you will tax from the lane minions. Usually if you do not get the kill get some minions but if you do get the kill it would be better to not touch any of them (unless your laner asks to) to prevent them from tilting.

If you get ahead of the enemy jungler you can start counter-jungling even without them revealing themselves. You will have level advantage and thus stronger smite to steal away camps. Or you can just 1v1 them in their own jungle and make them go AFK after. But be very careful about lane priority since getting collapsed and shut down can be detrimental. So do not get cocky.

A very helpful way to look at lanes is to unbind the option “Select Ally 1”, “Select Ally 2” etc. from the F-keys (F1, F2, F3, F4) and bind them in easier to press keys to be able take glances at your laners and gather info about the lane state, if it is gankable, etc.

When counter jungling try to place some form of vision deep into the jungle to be able to keep track of the enemy jungler, while protecting your team and giving you the opportunity to spot him and make some sort of play around the map either is an objective, a dive or even a simple gank.

PRE LvL 6:
Aim for lanes that your teamate has CC. Flank and start with auto attack for passive Unseen Threat slow. Then Q Taste Their Fear and orbwalk (auto attack, move beside enemy champion, auto attack, move beside enemy champion etc). When the enemy uses any dash/blink finish of with W Void Spike or use E Leap and finish of with Q, W and auto attack. If needed you can engage with E Leap but you risk the enemy getting away. Can be used in situation when enemy is very low and will die by a single combo, E Leap -> Q Taste Their Fear -> Smite for Electrocute or Dark Harvest.

POST LvL 6:
Aim for lanes that your teammate has CC. Flank and open up with auto attack for the slow, then Q Taste Their Fear and R Void Assault to prevent bursting you down and also resseting passive Unseen Threat. Use both R Void Assault charges. Then W Void Spike for healing, damage and slow if it is evolved. Save E Leap for when the enemy flashes or uses a dash or anything to gain distance and finish off with Q Taste Their Fear and auto attack. If a rare occasion occurs when the enemy has NO escape, open up with E Leap to surprise and get on top of them and it's the same for the rest.