Jax, like always, is a very binary character. He either has counters in the game who can survive his burst and deal damage in a way he doesn't defend against (Ryze, Poppy, ect) of he walks all over your team and you have to put two players against him late-game to keep him from murdering you under your turrets.
Basically, while everyone else in the game is playing League of Legends, Jax players get to enjoy a rousing game of Rock-Paper-Scissors. I don't find him particularly OP at the moment, but a rework is warranted nonetheless.