Plat V player who loves to play Jax in normals (tried him in ranked, but he's really not that good against competent players. Not bad, but he's not OP).
Jax is not that good at teamfights, he has to build 3 damage items (devourer, rageblade, gunblade), so even with ult, he is not that tanky. You will get burst down easy if you charge in the front-line. Jax is only good in teamfights if he can flank the enemy team.
He is a god in split-pushing. Assuming that he has rageblade and passive stacked up to max before a fight, he can 1 vs 2 easy. Do not fight Jax under your own turret if he already has his passive/rageblade at max stacks when you get there.
Important: You need to get to your own turret to defend against a jax splitpush BEFORE jax gets there. I've seen plenty of careless players (some of which are diamond players but get careless in normals). If you let Jax stack up his passive and rageblade before the fight, he WILL kill your under turret easy.