Well they nerfed Jax's base armor and mr stats so now he's quite papery in the early game which is why he he has all but vanished from the top lane, just can't really compete early on with trading.
Although I'm pretty sure he is considered a counter to Riven, purely because of his e negating that early damage Riven gets on her empowered auto-attacks, Now add in Jax's leap strike which allows Jax to stick to riven despite her dashes. Jax's w just allows him to win a trade harder than he already does in the match up provided that Jax was able to dodge the empowered autos and leap strike to riven after she blows her valor's dash and lets the shield decay.
Despite what I said though, I don't think Jax is quite viable overall. He's just too fragile in the early game and doesn't clear a wave as fast as other split pushers like Fiora or Tryndamere (Although man he shreds those turrets fast.) Jax may have a god-like late game, but the ramp up period to that late game period is too long.
Perhaps thats why Riven is complained about so much.The counters aren't able to do their jobs. But hey, now that Garen has seen the juggernaut changes and that Jax is getting some buffs on the PBE, maybe Riven won't be such a big deal when her buffed-counters start prowling back into the top lane.