played him a bit on the pbe. he's great in the jungle. on a turret, he's very lackluster. his mechanics are a bit tricky to manage in a fight though. he's very spammy but u need to have the right skills up at the right time to use his bonus' properly. if u want to save his root for when u need it, u cant use any other ability, which is an issue. if u spam abilities and go for max dmg, u probably wont have the root available when u most need it
he's also very snowbally. with kat losing her gold on passive, rengar takes the snowball crown. if he gets rolling, he's near unstoppable.
one issue ive found is that if he's not in the jungle, he's easily kited. he has no gap closer and no cc reduction if he's not near a bush. he needs to be in and out of a fight very quickly because in a long fight, anyone with a ranged cc will simply kite him to death.
i see him as a very good bot ganker. if u can win top, a rengar gank is a scary thing.