Build up ferocity on the creep to 4. At this state you're ready to start a gank, prepare for root or to heal yourself. Common mistake (in my opinion) that people build up 5 ferocity. Playing rengar is not about pressing Q and ret@rdjumping out of brushes. You need to plan your mooves or you'll be dead in no time.

Long story short, you need to get 4 ferocity points, than Bola an unlucky dude, so you can calmly Savagery the livings**t out of him/her. Of course keep in mind: cold beer, big boobs and brushes are your friends. You can solo top, ad carry and even mid with this badboy just keep in mind: this kitty is ain't no tank so keep your distance and join the fight for short bursts.

Pros: Insane dmg output, nice ganker, does well in the jungle, no mana, small cooldowns, brush fun, self heal speaks for itself.
Cons: Lack of defensive abilities, lot of people are allergic to cat fur. Oh yes... Vision wards and oracle elixir makes rengar a sad panda.