I bought him yesterday and was playing him for a majority of the day yesterday, both in dominion and in SR. He's very underwhelming so far, his only damage comes from his QQ combo (lulz) his slow is awful and is slightly longer than melee range, he has pretty bad utility, decent burst, not very survivable.
He has worse kiting issues than udyr, as he's completely reliant on brush for his gap closer and his incredibly long cd ult to catch people. Imagine udyr without bear stance, that's him outside of brush.
His unique resource is clunky and it's takes a while to generate ferocity in a fight, relying on being 4/5 at almost all times beforehand via minions.
His gap closer is very gimmicky, and is bugged to the point where you won't consistently jump to people in the same brush as you. His w is bugged as well where it removes your target, and my friend told me he had a game where his ult wouldn't even work.
He has very crappy 1v1 capability, and most of my kills came from people who who didn't know better and ran from me instead of dueling.
His ult cooldown is too long for what it does, and he's completely reliant on gap closing with it outside of brush.
So yeah. Take Riven, remove her shield, remove her multiple dashes, her multiple AOE hard CCs, give her a short ranged slow and make her require bushes to stick to targets. You now have Rengar.