Sion is not worthless. He is a powerful lane bully especially when first-bought items are either
with the addition of pots. Think about how the enemy laner is going to face you.
His shield allows him to trade practically for free - and I start it in almost all of my games. Focus on farming, and trading with your shield.
To poke, don't use your Q. It costs to much mana for the damage you'll get. Instead use Roar of the Slayer to poke constantly, and if the enemy tries to trade; use your shield, then roar of the slayer and follow up with Q. Your burst should discourage them from facing you like a man.
Your Q should strictly be used to deny CS or try to catch someone when they go to last hit with a weak knockup. (Which can let you follow up with Roar of the Slayer and move in with your shield for even more poke.)
I rarely almost ever lose lane as Sion, and transition into an extremely powerful late game tank with 5500 - 6000 HP and powerful endless CC.
I consider his passive useful, though exclusively in teamfights. Nothing is worse than killing a tank to see him come back and beat down 2 squishies that are trying to fish for kills past your turret. Soon everyone will understand how Sions passive works and they will walk away, but until that day comes I will be just fine engaging like a loose cannon and taking several people with me. He is not supposed to be a duelist. Period.
New Sion is not bad at all. Yes he is awful 1v1 outside of lane but that's not his purpose. Sion is a full tank. The very definition of a full tank - at 1350 IP to get people into the ideas of full tanks. Noone tanks better than Sion. Others may bring more utility than Sion, but he is the buttered bread of tanks now. Designed to let people follow up extremely well and disrupt the enemy team from picking up kills.
Stop playing him straight up AD, start AP and go for tankiness after that.
is the only viable AD Item on Sion and it is meant for better splitpushing. Sion excels at splitpushing as long as you have your ultimate. His wave-clear is insane, especially with a Hydra you can demolish a wave with your 3 abilities and Hydras active in under a second. If anyone comes to gank you just pop your R and run back up the lane. Unless it was a very elaborate gank with 5 people all waiting for you to show up at the beginning of the lane you were pushing - they should have no answer for an Ultimate escape. If they pick you with a gank then, you should have a monstrous amount of HP - ping for your team to come and help you or ping for them to take an objective. One dead Sion is not worth a tower/inhib or dragon.
I have not played Jungle Sion but I imagine it's more difficult as it relies on hitting Roar of the Slayer, you have almost nothing after that. That's kind of the cost of playing Jungle Sion though. Gank through the lane with your ult or suffer an easy escape by the enemy. His best ganking partner would probably be Taric, but good luck having that in every game.