I just played a game with Rengar on the PBE
basically, he seems like a horribly harassy champ in lane. jump->qe takes out a ton of their hp
can build bruiser very well (maw, frozen mallet, ga) and still do so much damage with fury.
you WANT to build the item that goes with him (the necklace) 20+2ad per level is too OP, and 2 stacks gets you +15 ms, which is pretty crazy imo. I can only imagine how fast he gets with ms quints/masteries
his ult is just icing on the cake, can assassinate people so easily. don't even need that to have a functional kit
but basically, rengar OP