My 8-0 3v3 team just lost our first match against two somewhat useless opponents and one overpowered rengar, who, from level one, would take out around 1/3 of anyone on our team's health in one poke. 7-0 rengar within first 10 minutes versus a shen top who had to build thornmail and randuin's omen to nullify a slight bit of his damage.
In 5v5, I see only a few problems. Rengar fits his role decently; if anything he could be considered a little on the weak side when put in top lane. His jungling however is incredibly fast -- finishing both golem buffs before the enemy jungler even has time to start red in most cases.
Overall Rengar plays most of his roles to a maximum -- something every new overpowered champion seems to do, except they are nerfed quickly in response. Perhaps I will buy Rengar next to easily get my team in the top 10.

bump

I cant undetstand this, maybe im a lil noob, but i know i can burst to death almost any champion, stack mercs, frozen mallet, bt, ie, sword of divine and could be black cleaver, lw or ghostblade, it works on my lvls, maybe in high elo rengar just dont works, but i can 3 hit a lot if ppl, not claiming being pro, im noob, but i think rengar isnt as bad as you think

Rengar isnt useless, but he is Underpowered, his damage is laughable at how low it actually is, most damage you can get from him is his heal, why is a heal doing the most damage? Riot logic...

He certainly needs a buff, better damage scaling, better armor and mr scaling, also for the love of god fix his Q its useless only good for jungling, E does more damage than Q which is another thing thats laughable....

On release he was an assassin - and did a good job of it. If they'd give him back his old damage with his current ultimate, I think he'd be fine.

At the moment, though, the only place I've seen him play well is as an assassin-support. Pet kitty for your carry.

His passive needs to deal additional damage starting at 50 + 15 physical per level when jumping at an enemy out of a bush or ulti imo

Build armor penetration on rengar.
Assassinate the squishies during teamfights (AP carry, AD carry).

Ive tried tanky rengar but it's very useless in teamfights.
You'll have to build him pure assassin, armor pen and attack damage.

I still love playing him in ranked lol