As one of the best Jungler right now, he have good damage and can have good sustain. Evolving the right Ability and know when to gank will give you great chance to snowball.

For stander Full Build
Pro:
1.Decent damage for all ability and able to help your ADC or AD top laner to do more damage as well.
2.Great health regen, once you get into late game, you won't always have to chance to recall and be in time for the next fight or contest for baron/dragon
3.Max CDR, able to let you poke with void spikes more often and the ability to escape with leap or engage more often
4.Randuin's active slow plus void spikes slow will help your team doing a bit more cc control which should allow your adc to kite easily.
Con:
1.Cannot resist pokes very well
2.Over the max CDR
3.May lack some vision control

The attack speed gain from mastery and runes will help you clear jungle camps a lot faster. By starting with Void Spike, it heals you well and it will allow you to only use 3 health pot instead of all 4 like most other jungler. Leveling the spike will also help you jungle at faster pace since reaching Level 6 will make Kha'Zix's gank much stronger

Ganking lanes should be pretty easy for Kha'Zix. As long as your laner is able to engage at the right time, you should be able to secure a kill or burn flash with half of their hp down. When level 6, killing champion without an ult that will allow them to live should die if nothing goes wrong.

This item will greatly increase damage if you want more damage instead of more sustain. You can trade out any of the item for this sweet
Also a great item to have. Helps you getting your health back up faster and gaining more team fight damage.
This is more of a risky buy but it is a great snowball item. It helps you do a lot more damage with the 20 Flat armor pen and the active allow you to chase easier without taking up too much time.
Gives good damage as well as survival against team that is more AP heavy. It will help your damage out put pretty well and greatly reduce magic damage.
Triforce is a great item on several champ and this cannot be ruled out for Kha'Zix. All the little stats help Kha'Zix do damage and also allow several extra ability cast as well as tiny bit more health. Try this item out when you are snowballing during early game.
Vision control is always needed, if you are against a high mobility team then get this item to secure vision.
Good against a poke or CC heavy team. The extra bonus health regen may even help you survive over time damages when you are escaping, etc...
The item will help since you are often in the center of the fight. It is also great against a AP heavy team as well.
Not a great item to build but if you are against a team that have quite a few high movement speed champion then this item may just turn out to be what you need. The little extra damage from Spell blade is nice too.