Guide updated 1 August 2013!
I'm a Swedish boy so please excuse all the grammar / vocabulary mistakes you can notice.
It's my first guide on season 3. I'm so it may lack of some information.
I have played Master Yi sometimes but now when the rework was done I Love him! I have just tried him in the jungle so it's just a jungle guide!

PROS

Pros
• High damages all game long
• Epic farming capabilities
• Good chasing capabilities
• Good jungling capabilities (epic if you have luck)
• Really easy to play against newbies / unorganized teams (resulting in a lof of fun =D)
• Interesting to play against skilled players (they will underestimate you)
• Opportunity to multikill (Pentakill ?!)
• Skilled baiting capabilities

CONS

• Squishy
• Low escape capabilities (if Highlander is on Cooldown)
• Heavily Focused on Team fights
• Hard to learn the right behaviour in Team fights (When to engage / run away), where to positionate)
• No special Counter-jungling capabilities
• Highly vulnerable to CC
• Hard to master if you are new but he shouldn't be so hard.

Let's start with Runes as it comes before anything else in the game.

Runes will give you an early "buff" as I call it
More damage, more mana, attack speed etc.

For quintessence I chooseQuint of Armor Penetration. I find out that it's best because it will help you kill that "op" tank in the end or beginning.

For the Mark I choose Mark of Attack Damage will boost your Attack Damage

For Seals I choose Seal of Armor. It will help you reduce damage.

For glyph I choose Glyph of Attack Speed. It will give you some attack speed boost that you actuality need!

This will be a short part about the Masteries chose
In this build you will go 21 0 9

Just regular points for Attack damage champions
Put 9 here for the extra moment speed, faster recall, reduced Summoner spells and longer 20% longer buff time!

Smite: Of course you need it ! Yi is too fragile to jungle without it.
Teleport: To get better. They wont even see you coming, you will be everywhere on the map its insane!

Other viable summoner spells
Exhaust: I can choose this because you can gang easier (it almost makes sure you grabb a kill early game), you can escape a tough situation in your jungle (someone going to counter-jungle you), you can reverse the tide of a 1v1 or a team-fight by disabling a major damage dealer and it gives you one CC ability.

Ghost: A lot of Jungle guide prefer this to have an escape/gank capability. I agree the movement speed bonus increase the surprise for the enemy, but it becomes useless when you learn Highlander. You can still take it if you feel more ***fortable.

Ignite: Somehow useful, prevents lifesteal for Die Hard (hello Dr. Mundo !) champions and increases the damages output while ganking. But I don't like it, if an enemy fled away with 10hp you made something wrong but it's np, you will catch him next time. It won't increase much your ganking possibilities IMHO.

Cleanse: Really useful, a spell I would take instead of Exhaust. It makes you far more efficient in teamfights and/or ganks. If you can't CC Yi you can't avoid to die. But you can get an improved Cleanse by getting a Quicksilver Sash instead. That's why I prefer Exhaust.

Heal: Why not but you already have Meditate and you may have some healing support champion in your team. Plus you should grabb Life Steal items so if you need life just hit the minions to recover some hit points. Heal can improves your baiting abilities coupled with Meditate. That would be the only useful use.

Flash: As for many of the champion, Flash is really useful to Yi. It can save your life or grab a kill sometimes easier than any other summoner spell. Ideal to tower dive or to jump through a wall. Coupled with Alpha Strike it gives a huge chasing capability.

This will be a short part!

Start with you QAlpha Strike, it will help you clear the blue camp fast and it's a nice ganking ability. Start maxing this Skill first!

Second skill to max is your E Wuju-Style. Passively increasing your Attack Damage. Activating Wuju-Style grants bonus TRUE DAMAGE on basic attacks, but the passive bonus is then lost while on cooldown.

Third skill will be your ultimate and I bet you all know what that do! Passive: If you kill or gain an assist it will reset your skills by 18 second or something like that... :p

And last skill is Meditate It will heal you and take reduced damage for a short time (While you are channeling)

Short part Too!

Berserker's Greaves will give you moment speed and attack damage standard on a AD champion like Yi

Spirit of the Elder Lizard will give you attack damage, Mana/Hp regen, dmg to monsters, 10% CD, And the passive on this item "incinerate" it's like an dream to activate your E it will make a Good true damage combo!

The BloodThirster will give you some extra attack damage and LIFE STEAL

Trinity Force will gain you some attack speed + crit chance and moment speed, slow your targets, more hp and attack damage, more mana and a bit more AP for you Meditate and of course Spellblade: after using an ability, your basic attack will deal 150% damage

Infinity Edge will give you more attack damage and 50% more critical damage!

Warmog's Armor for that extra hp you will need in team fights!

Sorry about the short information...

Jungle map
Start Blue for the mana regen and CD, then wolves, then go red if you want!

Wraiths Spawn time increased to 2:05 from 1:55

Wolves Spawn time increased to 2:05 from 1:55

Giant Wolf Base experience granted increased to 170 from 153

Golems Spawn time increased to 2:05 from 1:55

Big Golem Base experience granted increased to 160 from 137

Ancient Golem Base experience granted reduced to 260 from 340
Base Health reduced to 1400 from 1500

Lizard Elder Base experience granted reduced to 260 from 340
Base Health reduced to 1400 from 1500

Young Lizard Base experience granted increased to 50 from 10
Base Health increased to 400 from 300


Sorry short info here too!

This is a ward map, tell your support where too ward so you can gank better.
You can also buy wards to put at their red and blue for for counter jungling!

General
Health per level increased to 92 from 86
Base Mana reduced to 180 from 199
Mana per level increased to 42 from 36
Base Armor reduced to 15 from 16.3
Armor per level reduced to 3 from 3.7
Attack Speed per level reduced to 2.75% from 2.98%

Double Strike
Every 4th consecutive basic attack, Master Yi will attack twice, dealing 50% damage on the second strike

Alpha Strike
Master Yi becomes untargetable and dashes to up to 4 units, dealing 25/60/95/130/165 (+1.0 total Attack Damage) physical damage and 75/100/125/150/175 bonus damage to minions. Alpha Strike can critically strike for 60% of Master Yi's total Attack Damage.
Alpha Strike's cooldown is reduced by 1 second each time Master Yi basic attacks
Mana Cost: 70/80/90/100/110
Cooldown: 18/17/16/15/14

Meditate
Master Yi channels for 4 seconds, gaining 40/45/50/55/60% damage reduction and healing for 30/50/70/90/110 (+0.3 Ability Power) per second. This heal is increased by 1% for every 1% Health Master Yi is missing. The damage reduction is halved against turrets
Mana Cost: 50/65/80/95/110
Cooldown: 35

Wuju Style
Passive: Master Yi gains 7/9/11/13/15% Attack Damage
Active: Master Yi deals 10/15/20/25/30 (+0.1/0.125/0.15/0.175/0.2 total Attack Damage) true damage on hit for 5 seconds. Wuju Style's passive bonus is lost while the skill is on cooldown
Mana Cost: 0
Cooldown: 18/17/16/15/14

Highlander
Passive: When Master Yi kills a champion, the cooldowns on his basic abilities are reduced by 18 seconds (half for assists)
Active: Grants Master Yi 30/55/80% Attack Speed and 25/35/45% Movement Speed for 10 seconds. If Master Yi scores a kill or assist while Highlander is active, Highlander's duration is extended by 4 seconds
Mana Cost: 100
Cooldown: 75

If you have any questions about the guide just ask me!
And sorry about the little information it's like my first serious guide!
I hope you like'd it!