I don't care if Rengar's ult is stupidly weak right now, it fits him perfectly thematic-wise.
It's seems very reminiscent of "persistence hunting" to me.
If you don't know what that is, it's a style of hunting that ancient humans did. Basically, you would jog after a deer or whatever, tracking it for maybe miles, until it flat out dies of exhaustion. Flippin' savage, man.
See, with Rengar, if you're hunting someone (and they're not grouped up), they know you're following them, but there's little they can do about it cuz you're way faster than they are when you're under ult. So all they can do is run towards their teammates or run towards their base, hoping they get there before you get to them. I think it's great.
It is mostly a nerf, though, I'm not denying that.