Rengar is a very strong jungler with very high burst- and gank potential. To master him, you need to be good at fastklicking and timing your attack. With Rengar it's VERY important to see the perfect gank opportunity, because if you don't, it might cost you or your teammates a kill, or you will even get killed yourself. So it's very important to cooperate with your teammates and time your ganks perfectly. With his passive you have the abillity, that most of the champions don't have: surprising your opponent. The enemy won't do much, if you jump out of a bush, slow, or even oneshot him. In the lategame you will be able to oneshot the enemies thanks to your runes and full lethality items.

The most important thing on Dark Harvest Rengar is to farm your soul stacks and getting more and more AD. Most of the people underestimate Dark Harvest, or don't play it, because they don't know, how it works. So everytime you kill a large minion, creep or an enemy champion, you get a soul. Each soul gives you additional permanent ad. Sudden Impact is the perfect rune for Rengar, because the most powerfull thing in his entire gameplay is probably his ult. So with sudden impact he come out of stealth and gain even more AD. Eyeball Collection Is just additional AD and there is nothing better on Rengar than AD. So relentless hunter, because Rengar has nothing to dash into the lane and gank(expect the ult), so the extra MS will definetly help you supporting your teammates. Most people think, that the best side rune is propably the ultimate hat, which is not right at all. This is the most common trap, that new Rengar players fall for. Celerity and Waterwalking will give you even more MS, that you can combine with mobility boots and gank one lane after another.

So with this full lethality build you want to deal as much burst damage to enemie champions as possible. After you got your jungler item you want to get you duskblade as soon as posible. That's why i really recommend Poacher's dirk. Poacher's dirk is kinda a quest item, and if you complete the quest(approaching 4 large creeps from the enemie jungle), it will automatically transform into a serrated dirk, which is really awesome, because you literally just get the serrated dirk 600G cheaper. And it's actually very important, to back for your jungler item as soon as possible, because the redsmite will help you a lot in early ganks and fights with the enemy jungler.

With Rengar it's OK, if in earlygame you get pretty low in the camps. Don't be afraid and keep farming, if the enemy jungler isn't near. As a backup you should always have a smite to heal, when you are about to get executed. Always starting a camp with a leap out of the bush(passive) combined with your Q, because like this you will get 2 fury stacks instead of one. Only smite Drake or Baron. Don't smite without a reason or just because you want to finish the camp faster.

Pros:
*Very high burst potentional(specially with dark harvest)
*Very strong at level 3
*Very high gank potentional
Cons:
*No dash
*Pretty useless in teamfights without your ult
*Pretty hard to master

So at the end i want to say, that Rengar is a really strong champion. And if you master him and play very well with him, you can get fed very early in the game and already oneshot people at minute 15.