Can someone aid me with Rengar?

I used to like the old Rengar and thought I'll buy him from champion sale. He's fun and all, but I seriously feel that he's more of a one-hit wonder than he used to be. If your team is doing good and he gets ahead, he can snowball. But moment your enemies are similar in gold or even slightly ahead, he isn't very good, at least he feels not good to me. Personally I haven't seen a single Rengar do good after early game, when his teammates aren't snowballing also.

I just feel that he can't assassinate squishies as fast or as securely as some other champions. You get few kills early game, and you feel you're strong, but moment mid-late game comes up, you get beaten by same people you were bullying early game.

All the guides I've read are also some semi-tank-bruiser oriented. Basically armor penetration and then tank. So far that's the only build path that I've had some real success with him.