Pros/Cons:

+ Much Health (TANK)
+ Great Lategame with much damage and survivability.
+ Great Stun
+ Strong after level 6, when he gets hes beast ultimate (Cannibalism). Use it when at low health at minions, jungles to get your health up. If it's soon will be a team fight, save you ulti, it will make your team win the fight and you will pick up some nice kills.
+ Can play both AP/AD
+ Strong both duo- and solo-lane
+ Kill almost everyone 1v1 with Cryptic Gaze and high damage.
+ Pushing and farming realy good with hes Death's Caress.
+ Shield that absorbs (Death's Caress).
+ Hard to gank (Cryptic Gaze).
+ Will need many enemies to take him out.

- Weak against range early game.
- Pretty bad until level 6.

You can easy farm really good with a Death's Caress at maximum level.
Just go in with Death's Caress and press W to make the shield explode and see money falling in!
Late game you will one-hit minions which makes the farming really much easier!

Attack Speed Runes (Alacrity), AD runes (Strength), Armor Pen (Desolation) is really good runes for sion, With 100 crit chance at the end, these runes will increase you damage per hit and attack speed.