Yeah, I kinda like him now. He´s fun to play and can be effective in any lane and in the jungle.

Actually I found support to be a fitting role for Sion now. He´s got quite some power in lane and doesnt need many items to be useful, also its much easier to utilize on his early power with 2 people in lane rather than alone.
If playing as support, I buy targons and sight stone, after that I take Randuins when I can.

It synergizes very well with Sions kit, giving him much needed tankyness and a crazy good active, either used after initiating with R or after he died to bash some heads in chuck norris mode.

After all I think tanky items are the best choice on Sion, he´s got good AD ratios, but has no natural steroid and is easily kited in passive form, so dps items are only worthy, if you are and can stay ahead of your enemies - or if your team srsly lacks dmg.

I definetely prefer the tank build.
Sions got alot of short CD CC and is naturally tanky, they cannot ignore you for long, although they better do, cause theres no good in trying to kill Sion first.

Consider buying frozen mallet as your main and probably only offensive item: It makes escaping from Sion almost impossible, gives him even more nice CC in teamfights+tankyness and the best part is you being in your "I´m dead? so what? not much of a difference!" mode and slowing them to hell while dealing insane dmg.
Its usually not hard to escape from Sion in his rage mode, but with frozen mallet you can actually tear them down from full hp to 0 - sometimes that is.

Another Item I wanna mention is Zephyr: Its the 2nd offensive item I´d probably recommend for Sion. Dont buy it everytime, but if you need dmg, already have FM or you have at least one other tank in team, this items provides alot of great stats for Sion and gives you the freedom to buy other boots in late game.
Sion has got no steroid skill, so he really needs AS and also some dmg, but again: I suggest you build him tanky and with utility, if you have FM, you can crack some skulls after they killed you, and that will have taken quite some time then.

Sion is a full package of great utility and he´s tanky as ****, his dmg potential isnt to be taken lightly and he sure is fun to play.

But dont pick him blindly: Sion has got a hard time against mobile champs, everyone with a blink ability, also many snares and knockups/slows **** him up, as he usually is hard or even impossible to stop but also very predictable and straight forward.
Morgana is especially annoying, the shield doesnt prevent dmg from Q but the slow/knockup and her snare easily stops Sion in his tracks, especially so in passive form, as then he´s got no time at all to run around and evade skillshots, so either avoid picking Sion when they have many mobility champs/morgana or expect a huge challenge.