Updates:
5/3/11-First draft completed
5/3/11-Build Changes
5/4/11-Major Changes-Build items, Runes-See comments

Story:
The city-state of Zaun is a twisted realm of science, mercantilism, and magic run amok. While the young science of techmaturgy does much to advance civilization, its research and use in Zaun pushes the boundaries of morality on a daily basis. Zaun, choked with pollution from the countless factories and laboratories that spew waste into the environment, suffers greatly. As polluted as Zaun is above-ground, its subterranean levels are far worse. All of Zaun's runoff waste pooled together in its sewers, mixing into a toxic and mysterious concoction. A single plague rat rose from this alchemical nightmare - Twitch. Unlike his lesser evolved cousins, Twitch became as sentient as anyone else... provided they were as insane as he. A seeming byproduct of the evolutionary brew that spawned him was maniacal insanity, albeit in a functional form.
Twitch came to the League of Legends to represent himself "as the only one of his kind" on the Fields of Justice. Since then, he has found a generous benefactor in the city that gave him his evolutionary sentience - Zaun. Twitch is one of Zaun's premier champions in the League, and as such he fights for Zaun when political disputes between Valoran's city-states arise. Twitch has publicly stated that his goal is to gain enough influence and materiel from his work in the League to try to recreate the process by which he gained his sentience. While many challenges lie ahead in his quest - one of the largest being his own insanity - were he to succeed, it would mean a bold and brazen step forward in the evolution of all of Runeterra. Zaun has pledged its support for Twitch in his quest, though it remains to be seen what Zaun's ulterior motives are in the matter.
"The existence of Twitch proves that anything is possible on Runeterra." �- Heimerdinger
Tips:
Expunge has a large range, apply as many stacks of Deadly Venom as possible before using it.
Much like Evelynn's Shadow Walk , Twitch's Ambush has a timer. Watching the timer is very necessary when going stealth.
If your enemy is getting away, use Debilitating Poison to close the distance, allowing you to get the killing blow.
A very effective way of getting a kill during the laning phase is to drop a ward in the bush near the river and recall in their sight. Buy items and then use the summoner spell Teleport, then teleport to your ward in the bush and stealth out. Signal to your partner it's killing time.
Play cautiously around characters with crowd control. Twitch's low max health makes him especially vulnerable to bursts of damage.
Be careful of enemy champions who have vision Vision Wards or the Oracle's Elixir. This makes Twitch unable to scout safely and move close for a kill.
It is always good to use Ambush to scout out the bushes to look for enemies.
Spray And Pray is a very good ability to use during teamfights, because you can hit all 5 members of the opposing team.
Spray And Pray stacks extremely well with lifesteal much like Irelia's Transcendent Blades .
When the first unit hit is crit with Spray And Pray, all the units hit with that shot will also be crits.
Stacking attack speed can actually lead to less damage overall. Because his ultimate now has a limit on the number of shots fired, it is much more efficient to stack attack damage.
Build UsageEdit
Twitch's attack speed is among the highest in the game, try purchasing items with on-hit effects such as The Black Cleaver or Wit's End.
Using Lifesteal also helps Twitch's small health pool.
Combined with Spray And Pray, Twitch can take down a turret by himself.
If left unchecked by crowd control, Twitch can out lifesteal an entire opposing team with use of Spray And Pray.

Greater Mark of Desolation-Armor Penetration. Uh hello, this is Twitch.
Greater Seal of Resilience-Armor. Twitch can be squishy at the worst of times, a little armor-hand-holding.
Greater Glyph of Alacrity(7)-Attack Speed. Twitch is a poison, and you want to get as many poison stacks of the target as you can, ASAP.
Greater Glyph of Strength(2)-Damage. Just a little bit of damage to add with the Quints.
Greater Quintessence of Strength-Damage. Adds in with Twitch's ult very well, making him an unstoppable force if you buy Infinity Edge.

21/9/0
Offence: Twitch isn't a mage or a defensive Rammus type-Offence.
Defense: Twitch is quite squishy, no?
Utility: Has Mana Regen, but defense is more needed.

These are not in order. Follow the order in the build up top.

Vampiric Scepter: Twitch having lifesteal helps him stay alive early on, and is like a health regen substitute. Clashes well with his early on after-stealth attack speed boost as well.
Boots of Mobility: Twitch doesn't need to be TOO strategic speed-wise. He is a stealth, therefore if he has speed he can use it and the stealth to his advantage.
Executioner's Calling: I prefer it for Twitch, considering it is a Damage Over Time weapon, and will go together with his poison well.
Next Item is a Choice
Guardian Angel/Randuin's Omen: Take guardian angel if you are feeling a little "extra squishy", per say, and take Randuin's Omen otherwise, just as it clashes well with Twitch, and you don't need Guardian Angel if you aren't dying a ton, which you shouldn't be anyway.
The Black Cleaver: It gives some attack speed, but mostly you need the armor reducing passive-A good item for any Twitch.
Phantom Dancer: Gives you a bunch of stuff extremely useful for Twitch, and some more movemeny speed, which never hurts!
Infinity Edge: It gives Twitch the motherload, but especially the damage factor. This combined with your ultimate is a DEVASTATING combination.

The skill sequence is short and sweet. It makes your invisible first, your expunge next, and your slow last. You should just grab one rank of the slow early on, considering it at rank 1 slows quite well, and it isn't as important as your other spells.

Cleanse: It removes all of the icky-ickies that annoy Twitch so, killing him when he tries to run.
Flash: It helps with the quick getaways through walls, or the quick helps through walls. If flash just isn't your thing, though, substitute it for Exhaust.

Build Results: I have used this build 10+ times, and have gotten 40+ kills each times. Worth it, it really is.
Summary: Because of the above information, enough said.