I played about 800 Rengar games in norms and ranked in Season 5. Made it to Diamond 5 mostly playing Rengar jungle primary and top secondary. I copied the current op.gg item build from Akaadian and Reignover, but I haven't played much Rengar recently, mostly been playing Singed top at the moment.

Pros

+ Snowballs hard + High skill cap and fun to play + Great at roaming with his Thrill of the Hunt + Addictive passive

Cons

+ Weak to hard CC because he's squishy + Not the best team fighter, lacks targeted CC

You snowball very hard in the top lane if you get ahead because of your high damage and ability to gap close anyone with your Unseen Predator. A good technique to use when zoning lane opponents is to "drag" the wave to the bush so that the minions are closer to the bush and the enemy has to come closer to the bushes to CS properly, which will allow you to zone them from coming close. The way you drag the minions to the bush is as the minions are coming into the lane, take the aggro first and walk towards the bush slowly until the minions come very close to the bushes.
Play Video

The perfect combo, AKA the DELETION COMBO, consists of starting out with Thrill of the Hunt, Pressing your Empowered Savagery about a second before you jump, then Youmuu's Ghostblade active, then Bola Strike mid-air, then Ravenous Hydra and Battle Roar right before you land on them. Once you land you will autoattack, then you will have 5 ferocity again if you landed your bola, and you can Empowered Savagery again if needed. Most squishy targets only require the initial mid-air combo and 1 or 2 autoattack to kill, but if you need more then you can use more damage. Credit goes to Dekar for making this video.
Play Video

The perfect combo, AKA the LOCK DOWN DELETION COMBO, consist of starting out with Thrill of the Hunt, Youmuu's Ghostblade, Empowered Bola Strike in mid-air when you jump onto the target, and then Ravenous Hydra and Battle Roar when you are about to land and then autoattack, Savagery and your target should be locked down and deleted within 1.5 seconds. Credit goes to Dekar for making this video.
Play Video

+Contrary to popular belief that Rengar is an easy champion, he is actually one of the highest skill cap champions in the game preceding champions like Riven, Nidalee, Lee Sin, Yasuo and Vayne.
+Yes, his ultimate is easy to use and easy to learn the combos, BUT!, learning exactly what you're capable of in every single scenario possible takes lots and lots of games to master Rengar.
+You are a very high damage squishy that relies heavily on items and kills, so if you want to get good at Rengar, you must learn how to snowball in the majority of your games. This means, feeding and going even, ARE NOT GOOD ENOUGH, you must CARRY with Rengar because that's what he is designed to do. CARRY. You cannot go even and expect to win a game just because you didn't feed.
+Step 1 is learning how to not feed.
+Step 2 is learning how to get a lead and snowball off of it.
+Step 3 is learning great decision making when using your Thrill of the Hunt and Unseen Predator. One small misstep, one wrong move, can cost you the game easily, but master your judgement on Rengar and you will be an extremely strong assassin snowballing menace to the enemies.
+Your goal is to make your enemies FEAR YOU if they don't fear you, you are doing your job wrong. Rengar takes hundreds of games to understand how much power you truly have in your hands and how you can make the most of it with positioning, timing, and pure kill power. Understanding your kill potential in all situations will make you a very good rengar player.
+The BEST way to LEARN Rengar is to play Agressively and test your limits of how hard you can carry. Yes, you will have terrible games, but they provide a learning experience that is worth more than losing 1 game. Make those mistakes and grow as a Rengar player, and you will be happy when you get to the other side!

The easiest way to avoid being seen before ganking is to simply base and run straight through a lane because there are almost never wards deep in the lane in between towers. If you'd much rather gank from a different route, then be sure to have Oracle Lens so that you can scan before ganking.

Please note that the more Movement speed you have, the easier this will be to do. Also, these jumps take quite a bit of practice to get used to, so practice them. As you can see in the videos, the jumps took me several attempts, so they're not really practical, but they can come in handy some times.
Play Video Play Video
This teleport leap trick allows you to have an extra gap closer in tp ganks which is really useful
Note - This may not work due to recent ninja nerfs. So far, I have not been able to reproduce this as of patch 6.2
Play Video
This trick will allow you to cast all 3 of your basic abilities even if you have 3 or 4 ferocity already stacked up. It will help you to higher your dps and jungle faster.
Rengar ferocity stacking trick Play Video
This is an interesting interaction between Rengar when he takes a thresh lantern from inside of a bush. Credit goes to Waysor on youtube for figuring this trick out and allowing me to use his video for this guide.
Play Video

Play Video