I bought 
pretty early into the game for me and he was one of the first champions I actually picked up and knew how to play really well. I'm not trying to brag but I did so well with him I just recently realized he has a somehow very high skillcap. I can promise you I'd know if I was playing him badly and deluding myself, but I'm not. He just seems really easy and simple. Anyone care to tell me what's hard about playing him?
How to be good with rengar: land bolos, use bushes effectively, and avoid the frontline.
Basically its his decision making. Wish him it matters more, you put yourself at melee range and basically blow your one escape tool to lock someone down.
Rengar is pretty reliant on snowballing to be useful in games. That requires good decision making, knowledge of trading/matchups, map awareness, knowing when to roam, etc.
Go top lane.
Build path: Tiamat/Hydra, Whisper, Yommu, Mov Boots, Brutalizer.
Rengar one shot: 5 Stacks > Ult > Empowered Q > Leap > While in mid air Tiamat W, E > Auto attack > Q.
