I have used Sion support several times and have won about 75% of my games. I have not yet tried it in rank but he becomes and unstoppable CC crazy tank beast. Your ADC may be upset at first because he doesn't know what to expect. What he should know if that you go all in they need to KEEP ATTACKING. Your will absorb a ton of damage and be able to keep in the fight with your passive after you die. Did i mention the CC in fights? Also if you take the CDR route, you will be able to ult every 36 seconds or so late game. Mixed with teleport your map presence is insane.

See my win rate with this https://drive.google.com/file/d/0BxZauzHmidBUZHd0cm5mc09aUTA/view?usp=sharing

UPDATE: 10/22/14 - I now believe the CDR route is the better build. You are still tanky enough to do things as long as you max W first.

Exhaust, Flash if ADC has Flash, Heal

Exhaust, Heal if ADC has Flash, Ignite

Exhaust, Teleport if you go CDR for all the time map presence between that and your ult.

There are a few different sets that work well as far as combinations of Armor, HP, and MR. Anything with health will also help Sion deal more damage.

Reds: I take damage reds just to give him a little more umph. You could add more armor reds here, but then it will be harder to last hit with your Face of the Mountain support item in the early game.

Yellows: I feel he gets more out of the armor runes throughout the game vs health per level runes, but that would be your preference.

Blues: MR per level runes are best. Sion has enough health early game to not worry too much about magic damage at that point.

Quints: The HP/5 quints is my preferred because it keeps me in the early game much much longer. Armor quints and Health per level quints can also work out ok. Either quints or yellows should be armor.

I use a good balance of defense (17 points) and utility (13 points) so that I can be a tank beast and keep up with gold while supporting with wards and pinks as necessary.

To deal damage build health for your W and Atmas and/or MAW for good AD.

If you take the utility route dont worry about ad at all. Stay high health and fast on your feet. Negate damage, provide cc, and ward in the best possible places using that good movement speed.

The first four items are a must, the last two slots should be variables based on the situation. Remember that building health also builds damage for your W shield. Its easy to buy the early items, if you be make sure you have either around 400 or 900 gold so you can make a decent purchase.

If you can get Atmas and Maw in the same game you will deal some decent damage while still being very tanky but its better to be supportive.

If you get the 40% CDR, don't be afraid to use your ult for any situation. Saving turret, escaping, ganking, and straight up starting fights.

Q or W First

W or Q Second

R Third

E Fourth

Q is the best skill on Sion that will make or break team fights. Remember if u cant charge enough for the pop up that it still slows. Sometimes the slow is enough for a kill. I do W second for insane sustain while dealing damage. E I just use for armor reduction and another slow on the focused enemy to kill.

Your ult R can be used in so many ways. Great to get back in lane to save your turret, turn around a fight in a group battle (follow with charged Q and W shield), or even as an escape after you absorbed a million damage and they dont get the kill on you :)

UPDATE 10/22/14 - Yea...Max W first. Sustain/damage is better and the Q still pops up.

If your team knows how you operate, all they need to do is ATTACK when you get involved. You will keep the enemy team busy trying to stop you from all the CC and you will be able to absorb lots of damage while canceling thier ults with Q, negating thier damage with W plus items, and reducing the focused targets armor by 20% with E. You can always still ult at anytime for an additional pop up or escape if you didnt already ult in the fight in the first place. Even if you die in the end, if your team was aggressive enough, you should be able to kill anyone left behind with your passive.

Face of the mountain allows you to farm a little bit to increase your maximum health with your W passive while getting gold and health for you and your ADC. Early game you wont be able to farm much, but late game you will be able to split push with your W and if you get attacked just Ult to escape.

I dare you to try this out and tell me what you think.

https://drive.google.com/file/d/0BxZauzHmidBUZHd0cm5mc09aUTA/view?usp=sharing

Check out my success.