I just played Rengar on PBE. Granted, it was against bots and whatnot so it was incredibly easy to get stacks, but I think he's pretty absurd.
I ran like a super dumb movement speed build with like Trinity Force, Furor Mobility Boots, Zephyr etc. Even without the dumb build his mobility and catch potential are like second to none, mainly because of the power of bola strike.
1000+ range skillshot with an 80% slow for 2.5 seconds at max rank, which you can chain into a 1000+ range skillshot 1.75 second root, all while chaining bush jumps and using your ult... it's insane.
The mobility you get from getting any stacks on your Bonetooth is absurd.
The Battle Roar change combined with the movement speed after ult (or during ult as ell if you reach 20 stacks) actually makes him far more able to do his job of jumping on a squishy from stealth and getting the fuck out, without the frustration of being insta-bursted by triple-Q that your opponents felt.
His split pushing got hurt by attack speed being removed from non-ferocity Q, but his teamfight presence got a massive buff.
Probably won't play him in the jungle though. I really like the play pattern of the rework.
edit: from the game. again, against bots, stupid fed, whatever... but still, 650 movement speed is fun.
http://www.youtube.com/watch?v=GGauQAeRSAg