I have a Podcast Tutorial on playing Sion as a support, you can check it out here:
https://www.youtube.com/watch?v=isoV2Xdrqpk
It's kind of like a follow up video to another video on top of that, so check out Winning Condition's video too (link in the description on the YT page)
Well, shameless advertising aside, the new Sion is in a pretty rough spot. His strengths are difficult to point out, and can be incredibly hard to capitalize upon in certain situations. I explain it in the video but I'll explain it again briefly here as well.
The new Sion is a really great teamfighter. All of his skills are team-dependent, but landing them provides powerful utility to a fight. The new Sion seems to suck because people don't know how to play with him yet.
I'd say in solo queue, the new Sion is really low on the tier board. He's hard to play (though simple in mechanics), his playstyle is a little funky (AD caster with rather large cooldowns), it's hard to get random players on the same tempo when teamfighting, etc.
However, in an organized team, I'd say he's potentially a situational but very high tier pick. His skills can shred enemies and put several of them out of commission all at once if you can land them, his ult becomes incredibly scary when you have a teammate lock down enemies right before, and he has very serviceable poke as well, allowing you to slowly gain the upper hand even if the enemy team is being reluctant to teamfight.
I explain more in the video. If you want an example of a nice team play, check out this video (@1:49):
https://www.youtube.com/watch?v=tveyldeuI_8&src_vid=isoV2Xdrqpk&feature=iv&annotation_id=annotation_1795906997#t=1m49s
I ult in after Nocturne blinds all the enemies with his ult, giving me an easy shot and winning the teamfight nearly 4v5 (Zed was super low HP).