Pros:
-oneshot potential
-snowballs fast
-good in low elo
-just one combo to learn
-good damage scaling
-is a fcking tiger/lion/jaguar beast
Cons:
-squishy
-when he gets behind he is useless
-bad in high elo or against a clever team
-if he gets cced without full ferocity, you are pretty much dead
-cant do anything about tanks if not feeded
-is a "***** cat"

Rengar is really fun to play if ahead.
In low elo you get feeded pretty much every game, that means every game is fun!
He is easy to pick-up if you know the assassins bread and butter.
After just a few games ,you get used to his mechanics and start getting creativ.

Try to find your enemy jungler at the scuttle crab and kill him, maybe you will need help of top- or midlaner.
Gank a bit to get some free kills / boontoothnecklace-stack,
but don't get underlevelt so keep farming! You get really fast underlevelt when you focus on ganking so don't get manipulated by your pinging teammates and clear your jungle before ganking.

Now you want to get your passiv stacks.
It is pretty good if you already killed the enemy toplaner in earlygame because they usually get quiet tanky, but if not try to ks in teamfights or so.
Waiting at the enemy's buffs is a good idea if you got a teammate with you, but only do this if you are sure you two can take him down or you are ahead.

Don't forget to farm!!!
In teamfights you want to camp in side bushes with no wards in them (check the bush with red trinket) and the most important thing is like for every assassin to let your team or the enemy team ingage but NOT you.
If you ingage you will just explode.
Run behind the enemies with ult on and watch out for the stun abilities. the best moment is when tanks are busy fighting with your team, the sups are a bit away and burnt their cc or shield/heal spells and the best of all moments is when everything i mentioned before happens and the adc does such a thing like mf r or jhin r.
if the adc is dead or really far behind, your priority will be to destroy their midlaner or maybe, if feeded, their squishy jungler.
Be really carefull when you try to make catches because most of the item they wont walk around alone.

You can jungle faster and more efficient if you know a few tips!
Your first buff should die with a q so you have full ferocity, that makes you clear raptors/ wolfs way easier.
When you kill the Raptors you should pull the last one to the midlane and kill him ther with a q so you can keep your ferocity for the wolves.
If you start at the red buff you can wait at the bush that is not the one next to the raptors and when the buff timer says 8 you jump with q active onto the plant and go to the redbuff.
With that you will start the red with 2 fero and the q ready.
The best way to kill the frog is to e it and wait at the blue buff bush till it coms in jump range, then you jump on it with q active and kill it like other monsters.
You cant / its not usefull to try to keep that 1 fero from the wolves and go to the birds because they dont walk that far and usually you have just 1 fero left after killing wolves and jumping on the birds without any fero also gives you the 1 fero, that means just clear it and normaly walk to the raptors.
The same is with wolves and blue buff.

When you cleared some camps and you have fero left you can attack the plants to get your passiv timer reseted.
You can through warding escape chases if you have vision ont the krugs, you just have to be in the red buff brush and jump over the wall, that should help you escape.
You can jump on to the red buff from almost every bush around, that is very good when invading