The Pros of playing/maining Rengar, The Pridestalker.

Pros

Rengar has a high skill ceiling with many options on how to play him.
Rengar can win against almost anything hes thrown at. (Except Rammus, **** Rammus.)
He has some pretty sick looking skins, for example, Night Hunter or Mecha.
Rengar can literally be played in any lane, with varying difficulties for each.
Very, very satisfying to pull off kills with.

Cons

Rengar's entire ult combo can be nullified by using Zhonya's Hourglass or Stopwatch.
Rengar has difficulty fighting champions like Darius or Rammus.
Rengar has alot of hate from the community given he one shots almost everyone.
Rengar doesn't really have as many voice lines as other newer champs. (Please riot give him new ones to fit with Ixtal existing now)
Rengar's default model is only slightly outdated looking. (I personally wish he looked more vicious, blood dripping from his knife and stuff.)

Unseen Predator

Rengars Passive is Unseen Predator, allowing him to leap to targeted enemies with his auto-attack, if hes in a bush, or stealthed. Rengar's abilities generate Ferocity when used. Leaping while at 0 Ferocity generates 1 Ferocity. At 4 Ferocity, his next ability is empowered. Casting an empowered ability increases Rengar's Movement Speed for 1.5 seconds. Letting him chase down kills. "Lesser creature."

Savagery

Savagery is Rengar's Q ability. This is his main damage dealer and Fury builder. Rengar gains bonus attack speed on his next two attacks when used, and the first attack does 30 (+100 AD) to 150 (120% AD) bonus damage. With 4 Ferocity Savagery's damage is increased and grants Rengar Attack Speed for 5 seconds. "Carve off a souvenir."

Battle Roar

Battle Roar is Rengar's W ability, and it grants him a built in QSS, and sustain option. When used it deals magic damage to nearby enemies and heals Rengar for 50% of the damage he took in the last 1.5 seconds (75% from monster attacks). With Ferocity it deals increased magic damage, does the normal heal, and breaks Rengar free from crowd control. (Not Suppression) "Come on!"

Bola Strike

Bola Strike is Rengar's E ability. When used it throws a skillshot bola, that slows enemies hit, dealing a tiny bit of damage as well. The slow lasts 1.75 seconds at all levels. But increases with levels, from 30%, to 45%, to 60%, 75%, and finally 90%. When empowered it does a bit more damage, and roots instead. for 1.75 seconds. "Slow down!"

Thrill of the Hunt

Rengars Ultimate is Thrill of the Hunt. On use, Rengar gains massive movement speed and True Sight of the nearest enemy champion within range. After 2 seconds Rengar becomes Stealthed, and can leap to an enemy with his auto attack. Leaping to the targetted champion deals a bonus +50% AD to the target and shreds their armor for 4 seconds. "Focus..."

Jungle Rengar

Rengar is a jungle monster. Capable of slicing and dicing through camps in split seconds given enough prep. He excels at early skirmishes against weaker early junglers like Master Yi or Evelynn.

Rengar has alot of tricks up his proverbial sleeves to get him ahead. Including the first trick I'll show you. The early ferocity jumps. These are meant to give you 2 stacks before you start red, and as soon as you do you get 3.

This is where you blast jump.

At the exact moment of 1:24 minutes in, you jump and use Savagery in mid air, to gain 2 stacks before even fighting. Then, take your red. You'll get empowered q off it as well. Use that if you get a bad leash. If not, save it for this next trick.

This is the Krug jump.

If you want to save your ward, let your laners walk past it so you can jump to it. If not, ward it and jump with your empowered q. Its not needed but it makes it much faster. You want to get Battle Roar off of level 2 to make Krug clear and Raptor much easier. Use your empowered Battle Roar as much as you can to save on health.

Pathing afterwards

After you take Raptors. (Recommend you smite big Raptor and kill it first) Go for Wolves, or an early gank on mid.
Once Wolves are done, try to take top/bot scuttle.
If the enemy Jungler is there, and they don't have help from Top/Mid, try to kill them. The only real junglers that can win this confrontation is Nunu & Willump, Warwick (Depends on his health), Ekko (If good), and Sett.