Sion is really strong if you know how to play him. he offers stronger teamfight potential than even a Malphite or Kennen, but only if you play him well. In the jungle, Sion can farm HP way quicker than in toplane, and you're in a stronger position to impact the map with your R. Sion is probably the champion with the least gold dependency because he has a lot of CC and permanently increases his health every time he kills a unit.
This build is what I like to call a "Full Health" build which builds items with a ridiculously large amount of health, in order to utilize Titanic Hydra to make you hit like a truck, and Gargoyle Stoneplate to make you able to withstand a truck.
This build is very durable, offers high damage, and prevents targets from escaping you. However, all this upside comes with the big downside of not being able to contribute much until you get at a minimum your R and Mythic (Frostfire Gauntlets).
For your first clear, you generally want to just start on bot side for a good leash. As Sion you want to full clear, as you couldn't contest scuttle when it was super valuable and now that doesn't even matter. Plus in a full clear you generate about 150 permanent extra Max Health. Make sure to back when you can get a Bami's Cinder as it significantly increases clear speed early on, and more clearing equals more health.

Sion jungle is really good if you have a team with a lot of upfront damage and a little bit of CC for really strong lockdown. Typically Sion synergies with burst mages a lot as they usually have a CC spell and a ton of damage to kill the enemy. Assassins also work really well, but they typically lack the all important CC, but they make up for it by having usually more damage.
Sion does not synergize with a tank in the top lane, as both of them have a lot of CC and overlap too much. The only way for them to really work together is if you have a high damage mid laner and a carry support. Sion's worst nightmare is having a Malphite top, Galio mid, and a Leona support (adcs are mostly all the same, damage). If you are determined to play Sion with a team like that, play Lethality or just play another jungler. Another thing Sion doesn't like are scalers with no gank setup, think Kayle or Kassadin.
Sion is generally fine with bruisers, adcs (ones like Draven are really good for him), and enchanters.