Pros:
-Twitch is an invis character
-Twitch can give your team huge advantages in team fights
-Twitch does decent amounts of magic damage to go with his insane physical damage
Cons:
-Twitch is extremely weak early game
-Team screams at you for ksing a lot
-High risk of penta kill late game

For starters there are a few things to be aware of while playing Twitch. First Twitch is squishy slow and has very low damage potential early game. He's not useless buy hes not going to nuke somebody down with a couple of 1337 crits either thus teamwork is imperative. Second is that Twitch is hugely gold dependent so successful late game team fights rely on you getting 75% of the creep kills in your lane.

For runes Twitch is pretty versatile because the only runes that are useful for him late game are armour pen runes. Although I chose to go with health quints, dodge and cd reduction to add to defense and reduce the time it takes to re-stealth to fill in the remaining 21 rune slots. It's all about preference.

Masteries are a bit more complicated. I suggest taking 21/9/0 as opposed to 21/0/9 simply because greed, health regen, and reduced time dead arn't as useful as extra magic resist dodge and the chance gain run speed if you dodge. The extra defenses help prolong your death long enough in team fights to start mowing down the enemy team and the chance to increase your run speed is nice when you realize you are getting eaten alive by Jax, Kayle, Kog, or whoever might be calling the exterminator.

Items are the hardest part.
To start off you will have low health, next to zero damage and extremely low attack speed so you have several choices. Doran's blade or shield with a health pot or my preference vamp scepter. If you go with a doran's item your best route afterward is to build Avarice blade > boots of mobility (ninja tabi if the opposing team is melee heavy) > 2nd Avarice blade (the avarice blades are cheap and pay themselves off by late game where you sell them to gain room for better items) If you chose yo go with a vamp scepter and risked the lower health for more life steal (aka infinite health pot) you should build that into an Executioners simply because its also cheap, it will give you the early game crit chance that you need to gank and with an added ignite debuff unfortunately without the damage although its still useful when fighting a Vlad or a Mundo. After that follow the item list above until your last 2 items because late game is highly situational. If you're fighting a high health high armour team go with a Madreds Bloodrazor and a Black Cleaver. If you're fighting a squishy nuke team go with a Phantom Dancer and a Blood Thirster. Common Sense people.