It's very, very easy to tell when Rengar is going to ult you. He jumps at you, chunks you to 50%, then suddenly prioritizes going off screen? Most of the time, I just activate R while I'm fighting and have it go off as his Q steroid ends. Going back gives too much time to react and possibly make you waste your ult which you can't afford to waste (140 second cooldown? Seriously?)
Maxing E is good vs. people you know you can zone, but for others it's not going to do much since the slow doesn't improve much per level. Most of the time, you'd be able to zone them by baiting their burst into a maxed W and run by slowing them after they dash/CC you.
The worst thing about maxing E, is that if the enemy is really sticky (like Irelia or Xin), you have to have an empowered E up to be able to get away, but that uses up your sustain while they have passive sustain on other sources, meaning they will ALWAYS come out on top.
Also, his first blood and snowball ability were neutered a long time ago. Remember double jump Rengar? That was so incredibly fun that I just feel like Rengar's missing a part of his soul without it, much like how Alistar doesn't function nearly as well without knowing his headbutt->pulverize combo.
Maybe it can return as part of bonetooth? That way it doesn't get out of hand early game, but still allows him to counter-zone those freaking annoying ranged top champions like Teemo/Jayce/Elise while maintaing relatively poor dueling capabilities vs. other common tops.