Those who have tried it say it works (so far).

This guide isn't too hard, nor is Sion. However, I'd like to mention that melee champs can get hard at times, so please don't ragequit the second you get a ranged top, or someone with a lot of poke. Sion may be harder to learn for some than others, but I feel that with a little practice anyone can be a pro with an underestimated champ.

Terms often used:
TL;DR- Too Long, Didn't Read (Summary)
MR- Magic Resist
HP- Health Points
Pen- Penetration (armor, typically)
Regen- Regeneration
Playstyle: The way you play champs

Pros:

Awesome mid-late game
Doesn't require lots of skill
Builds damage for a good time
Plenty of survivability in fights
Totally Annihilates in 1v2's, 2v3's, and sometimes 1v3's
Ignores Damage as a passive

Cons:

Rough early game
Takes some practice
Short stun duration
Hard to chase with
Lame in-game appearance (to some)

TL;DR: He's a little hard early, but really easy late.

MARKS:
First Choice: Attack Speed makes for GREAT utility, all the way through the game. It really works with his Enrage and Cannibalism.
Second Choice: Damage. Damage is always good, whether it be scaling or not.

SEALS:
First Choice: .5% Max HP really stacks perfectly with his Enrage, and also gives you a great reason to rush Atma's Impaler.
Second Choice: Armor. Protect your HP.

GLYPHS:
First Choice: Magic Resist. Protect that HP.
Second Choice: Ability Power. Now, I said wtf at first too. But his Death's Caress, aka his shield, really scales well with AP. This can really help your early game, and while I wouldn't recommend it being your first choice, if it's what you have it's definitely a viable option.

QUINTESSENCES:
First Choice: 1.5% HP is just.. Fun.
Second Choice(s)*: Attack Speed, Armor Pen (Per Level), Armor (Per Level), MR (Per Level), Movement Speed, Mana Regen, HP Regen (Per Level), or Lifesteal. This is up to you, and these are the only quintessences I see as viable options on AD Sion. Remember, Runes are your utility, and can be used for either making you do tons of damage, or better balancing your build. I prefer the balancing options, but if you decide to rush huge damage early on, that's your playstyle, and I can't say it's not good if you can get it to work, but I will state that the viability of this guide is playstyle dependent and therefore requires some general understanding of Sion to work. Practice makes perfect.

TL;DR: Attack speed is good, %hp increases are REALLY good late game, lifesteal can be a good option, and glyphs will most likely just be MR.

I use the same Masteries for Sion as I do for Dr. Mundo. Just remember that. Also, I'd like to mention that Masteries are a personal preference, and highly subject to alteration as seen fit.

The Attack speed just plain works on Sion. Try it. Having Destruction (5% damage increase to turrets) is just a personal preference as a utility, I find it to be useful. Deadliness and Destruction masteries are subject to change, whereas, I believe Fury , is not.
Durability is a very good tool, especially since it helps you get Veteran's Scars , another mastery set that works well on Sion. Perseverance is a personal preference I find handy, Hardiness and Resistance work well (although Resistance is subject to alteration). Safeguard is yet another personal preference, as I like to 1v1 under their turret when I have ult and Ravenous Hydra. Why?? Because I can. Besides, they don't expect me to live AND get the kill, both of which are easily possible. Juggernaut , in my eyes, is a must-have, as well as Reinforced Armor (to some extent). The rest is quite optional, and again, subject to alteration as you see fit.

TL;DR: Just read the mastery chart and copy it as you see fit.

My playstyle for Sion, as you may have noticed, is unlike most, and therefore I pick Exhaust over Ghost. In early game initiates, which sometimes happen, (hence why I occasionally pick Dagger as a starter,) it's easier to get their health lower by slowing them down instead of speeding you up, thus granting you more time to exchange before their turret, as well as reducing their damage output, and making them easier to kill if you change your masteries slightly. I'm not saying Flash or Ghost are bad, however. Using them as an escape is fine, but I still think it's pretty easy to pick someone off in a teamfight using Exhaust, as well as escape if you Exhaust the right champ. It takes some skill, but I still think Exhaust is the better option. As for your secondary, Teleport or Ignite work well, depending on your playstyle, or even Barrier or Cleanse, all according to preference.

TL;DR: Exhaust can substitute ghost and flash if you're good, and ignite, teleport, and barrier are all good options for a secondary.

The point of Sion's Enrage is to get huge HP early. Also, on a sidenote, the reason AP Sion works is because his Enrage only has to be active at the time of a kill for him to gain HP. It could be a kill made by his shield blowing up, just as long as his Enrage is toggled on, he'll receive HP. Fantastic for farmers, especially AD farmers who benefit from the excess damage. Therefore, I always get my Enrage first. Now, his Death's Caress has a little more priority than his Cryptic Gaze, in my opinion, if not slightly more. Farming has a great impact on which skill you're getting, but again, his Enrage is always a great rush. Now, as for his Death's Caress, this ability is a shield, and it can damage nearby enemies if deployed correctly. What more could you want? Oh, you could want a stun. Death's Caress is for if you're not having too much trouble at level 1. But if you know they're poking really hard, chasing a lot, or if they just plain have the upper hand, you might want to level your Cryptic Gaze second. "Why would I need a stun at level 2?" Escapes. We all know Master Yi enjoys his Alpha Strike. We all know that there's going to be a Singed with Exhaust and Ghost, who gets his Mega Adhesive at level 2, therefore you have a stun to escape with should you need it. Now, as for level 3, I'd recommend your Enrage. "Wait, isn't Cryptic Gaze/ Death's Caress more useful?" Early game? Sure. But here's the thing about Sion. You're not going to dominate your opponents at level 3. I don't care if you've perfectly last hit every minion and gotten first blood. Unless you have Ravenous Hydra, DO NOT ENGAGE. Your Enrage is for farming early game, and what do you want to do early? Farm. Poking isn't needed. Pushing is fine if you're prepared for a face full of jungler, and for those who level their Cryptic Gaze early and get Ghost. But for the purpose of this guide, I'm going to ignore people who initiate at level 3. I'd say, sure, get all your 3 abilities at level 3, but I personally wouldn't. That's just me, take whatever you want out of this guide. At level 4, I have 2 on Enrage, and 1 Cryptic Gaze and 1 Death's Caress. Seems to work quite well. Prioritize Enrage, then Death's Caress, then Cryptic Gaze.

TL;DR: E->W->E->Q works best unless you're being poked and tormented like hell.

Yes, we've all seen it. There's always a time when you're stuck 2v1 at top because the enemy team decided not to have a jungler. The good thing is, their late game is gonna suck. The bad thing? Your early game already sucks, and now you're 2v1 of all things. In this situation, I still get my Enrage first, but you have to exercise much more caution when farming. This is when I'd recommend getting health potions, teleport can also be useful in situations like these especially. Now, as for your second spell, perhaps get your Death's Caress priority over Cryptic Gaze. I realize that in my previous chapter, 1v1 Top, I said Cryptic Gaze is good for escapes. However, unless it's only one following you, you have to remember that you can't stun 2 champs at once. This is the occasion that I'd level Cryptic Gaze third, and Enrage fourth. Otherwise, always always always prioritize Enrage, then Death's Caress, then Cryptic Gaze.

TL;DR: Dang it I'm 2v1 top, time to go E->W->Q->E

Sion is a secret weapon. Treat him as such. His Cannibalism can also heal allies as long as you're fighting. This is very useful. In teamfights, only initiate if your stun and ultimate are up, also, preferably your shield as well. If you're second in a fight, pick off a big damage dealer with your stun, then focus someone squishy, whether it be the same champion or not. Also, sometimes it's a good idea to focus tanks. Why? Because as long as their team is grouped, you'll still get plenty of hp, especially from the tank, and the enemy team will try to focus you, failing as long as you've used your ult and have enough attack speed to sustain yourself.

TL;DR: His ult can heal and you've got tons of damage. Run in and stun whoever you well please if you're too lazy to read that.^

Ravenous Hydra is your rush. Get it.
Lifesteal can be nice, but his ultimate has that built in. What you really need is a little movement, some attack speed, some damage, and crits/MR/armor as you see fit. Phantom Dancer is a much better item than Zephyr due to the crits, higher attack speed, and overall similar bonuses with better stats. Zephyr can still work, and is great vs CC heavy champs, but I'd typically recommend Phantom Dancer first. Bloodthirster can be a good early rush for the huge damage and lifesteal addition, as lifesteal is good, just not necessary. Some lifesteal is recommended, though, because you don't want to be Warwick, the walking ultimate. When your ultimate is down, you still need some survivability. As for Attack Speed, an explanation of why it's so op is as follows:
Your ultimate heals nearby allies. Ravenous Hydra hurts nearby enemies. The quicker you can get your damage dealt out, the more health you can get to everyone. You're a tanky AD support in a sense. This is the reason Zeke's Herald and Black Cleaver can work. Armor Penetration is also at Reader Discretion.

TL;DR: Get some attack speed, damage, and crits/lifesteal/armor pen as you see fit, along with whatever survivability you choose.

I have not tried this Guide in ranked play, and therefore would recommend Reader Discretion when attempting ranked with Sion in this form.

Build damage and some lifesteal. As long as you've got Ravenous Hydra, the rest of your build is situational, making him really fun to play around with. After a game or two, you'll be dominating as Sion, the AD Destroyer.