Why play sion?
´First of all: Sion is awesome!
Having said that, he's a good and persistant damage dealer, who can slightly disable importants parts of the opponent team while helping his own team by healing them with his "r" of death and destruction!

When playing and AD sion, it's important to get the items fast!
Therefore i believe that tiremat is a much needed items, because it makes your farming so much faster, which also grants you a huge amount of health in early lvls, makin health items unneeded. FoN is also an important item to rush, because sion's passive makes him very persistent to ad attacks, but rather fragile against Ap. Despite his passive, the dodge shoes gives you a great advantage when countering ad champs and even carries. And in the end when you got your build, there will be noone stading in your way and if you don't feel like dying just press "r":)

I dont' believe that my choice of runes are the only one usable. There are a lot of different runes that would be just as great as those i have chosen with sion, fx dodges runes could be nice or maybe ad runes. Nevertheless, armor pen runes gives your insane base damage a nice impact on your opponents health, and armor + magic resis grants you enough persistance to chop your enemies down:)

As in my runes, i don't believe that my mastery setup is the answer to making sion a God, but getting 21 in offensive and 9 in defensive makes you a nice persistent damage dealer who can survive mostly anything thrown into his way. On the same level you got a shield which also can block some critical damage.

Again an open subject: But i prefer flash and exhaust. Flash because it works very well with your stun, both for getting away and starting a fight. Exhaust because it makes any carry champ ran home crying into their mothers arms and when it is in combination with your ulti you'll be almost immortal.

Pros:
Persistance
Tough
Disabler
Huge Dps
Tean assisting
Late game BAWS! CHOPSTER!

Cons:
Low ranged
Farm dependend
Old animation:)
Early game weakly!

FARMING IS SO IMPORTANT!!!
Therefore get tiamat and sell it before buying another phantom dancer! Tiamat does not only work great when farming a lane, it also does a great job in teamfights, because it grans you a lot of lifesteal when your team is in trouble and you pop your "r". This way you can actually actually heal your team while dealing a ton of damage to the enemy team and if they stack up to kill you, your splash damage will be davastating.

When you lane as AD Sion you shouldn't play very agressive. Actually you should focus on farming becuase Sion is really an late game game champion and won't be able to do very much early game. Nevertheless, many champions play offensive and to counter those you can stun them and hit them a couple of times which scare most people away because of the huge base damage you deal but if the enemy champion stays have your exhaust and flash ready.

Well you can do almost any skill order you like with sion, without causing any real harm to yourself. I prefer maxing me "e" and "w". This is do to max my health farming at early lvl's making me difficult to kill allready early game and late game almost impossible. The only problem with choosing "e" as your first choice is that it does deal a fair amount of damage to you, but this can be outdone by lifesteal and dorans blade therefore does a great job. WHen you have one lvl in gaze "q" it only scales on damage and that's why i only max it late game, because maxing your shield secondly can give you a bit more survivability and assists you in not killing yourself with your "e".

I think it's important when playing any champion that you consider the enemy team when building your items. My items is in my opinion the best with sion in generel. BUT, if you face a team with mainly AD or Ap you should change it a bit. If i play against like 4 ad champs i prefer thorn mail instead of Force of nature and if you face a mainly AP team you should get merc treads instead of dodge boots. Merc treads are also very nice against a hard cc(crowd control) teams and if you lack real ad DPS on your team, it's also nicely considered by me takin AS boots.

When you play Sion, it's very important to farm and if you are like me you sometimes tend to focus on farming instead of being in every teamfight. This is surely a problem but i believe that if you know that your team wont die or can hold of the enemy team, it is very important for you to farm as much as possible. So people who team up with ad sions, dont be afraid of them not being in every teamfight, they will carry your team to victory later on! You really need to farm, i cannot underline how important it is, so sometimes you got to ignore haters and noobs who doesn't know what you're doing. That's sadly a part of playing Sion.

When you have farmed and your team haven't complained to much and you finally get into teamfights, you'll have moments like these:

The entire opponent team is doing baron, I (playing Sion) choose to charge the entire team because them getting baron buff could be game turning. The idea seemed kind of stupid but sometimes you got to try something stupid. When i arrived at baron i got caught by Skarner's ulti and got supressed ending up in the middle of the entire team. In that moment i had allready given up and didn't have my fingers on the keyboard anymore, but then i realised that they were out of cc. So i stunned their carry, pressed "r" took baron with my splash and got 10 seconds later i had myself a quartable kill:)

Another awesome moment is also picking up fights with sion and another dps tanky champion. In this case i picked up a fight with the entire opponent team with me (playing sion) and a mordekaiser. We knew we couldn't run anyway, so we'd rather stay and fight. I used my "r" as fast as possible and thinking morde did most damage the opponent team focused him with all their cc. A very big fault by them, because i could heal him up with my insane damage and splash and with an ulti as nice as mordekaisers we won the fight and got our self three kills. They then realised that they couldn't win and surrendered. So sometimes you gotta pick up fights when you know you can't run.

Sry:(