Rengar is still strong, one of the higher-tier (Not top tier) junglers.
His most popular build atm is a bruiser build that sacrifices burst for durability.
You can build all Lethality items and one-shot people. His main weakness is that he has no way out, unlike other Assassins that have resets or leaps or blinks to escape a fight after an assassination. If you want to play burst assassin Rengar, make sure you have a team to back you up and a tank to initiate for you.
His W is better than it was before, not nerfed. It doesn't give resistances, but it's healing scales with the amount of damage you've recently taken. If you take a lot of burst at the beginning of a fight, but survive, hitting W will heal you for most of the damage you took. This is represented by a grey bar in your health bar.
Q is a skillshot now, so just get better with hitting it. It's an active skillshot to discourage priming it while stealthed or while in midair, so his burst is not instantaneous upon leaping.
His new Bonetooth Necklace is also very important. You need to get a takedown on every individual member of the enemy team to get to your maximum potential. Full Lethality builds with Infinity Edge and 5 Trophies will delete any squishy.