Master Yi Build
Not Updated For Current Season
Master Yi is one of those characters that a lot of people buy with their first 450 IP and then throw away once they get someone who costs more. Unfortunately, these people miss out, as Yi can be a very powerful character.
Even though Yi can really pull out the damage, he really isn't an instigator to battle. Even with Alpha Strike, you shouldn't be rushing in first. With only 1992 health you are lucky if you last even 3 seconds. Instead, Yi is almost a closer of sorts. Alpha Strike can hit a good chunk of their team for enough damage to take out a severely weakened fleeing player, or at least put them in a one hit range. Highlander is an extremely effective chasing tool, and if timed properly, can be used to take out an entire team. If you are mindful of Yi's extreme fragility, he can be used to execute almost instant kills in a gank, chase enemies down, cause the enemies to flee, and farm massive quantities of gold while harassing the enemy.
Start off with Wuju style, and make sure your laning partner has a slow/silence or is a caster/ranged. Ryze has been good for me before. If you don't have a partner with a stun or slow, then do not try and chase. You do not have a stun/slow/silence yourself, and chasing will just get you killed. What is more effective instead is to wait until there are 2-3 minions left in the wave and use Alpha Strike on the one closest to you that still leaves your opponents within the area of effect. If executed properly, this will allow the ability to hit for around 1/4 or 1/5 of their health without putting you within hit range. Using this, and occasionally popping Wuju style if they decide to get a little fresh, you can easily get a kill, or at least get enough gold for the berserker's greaves and the brutalizer.
Mid Game: By now you should be around level 8-9, and have, if you are careful enough and good enough with the placement of the Alpha Strike, Youmuu's Ghostblade (Or at least a good chunk of it.) Right now, you can easily put out more damage than any other summoner. However, please don't. If you get slowed, it will be all over, and more importantly, you are so squishy that even a slightly fed champion will crush you. Instead, always travel with a partner (duh), and if the partner is not pulling his weight, or there is a gank and your partner goes down immediately, use highlander and high-tail it out of there. Your number one goal at this point isn't to get kills, it is to make sure you aren't feeding. Alpha strike will be your greatest weapon in getting gold, allowing you to take down entire waves of creeps very quickly. Use it to your advantage. Farm gold, get Zeal and sword of the Divine. By all means, if your team decides to push, get aggressive and take down enemies. By level 11, your Highlander will make you hit ridiculously fast, your attacks will crit very often, and you have the best mobility on the field. Use it to your advantage.
Late Game: Hopefully you will have most or all of sword of the divine, which is where this Yi starts gaining some real power. You may not have the best defensive capabilities, nor will you have the best damage per attack, but you will hit fast, crit often, and chase well. At this point, there should be team battles and team pushes often. Make sure to travel the jungle with a partner, as even two enemies will be able to take you down. In team fights, never instigate combat. Instead, wait to see who the instigator targets, and charge up to them without using an ability, and use Highlander, and then Wuju Style. If you get the kill for the enemy, run back a step and scan the field for the enemy with the lowest health. Alpha Strike towards them and then continue attacking them until highlander runs out, then re-apply both highlander and wuju style. Rinse-Repeat until you run out of mana, which will definitely happen. If you only get the assist (unlikely with your attack speed) find the lowest health enemy, run back, alpha strike, then attack the lowest health enemy. If you start taking damage, run back and wait for Highlander to cool down.By this point either a few of their team have died, or your team is weakened/dead. If you are winning, start chasing them, using highlander and alpha strike to get closer and pick them off. If you are losing get the hell out of there. You are no hero. If you are successful, get Phantom dancer and whatever the hell you want for a last item, preferably a Stinger, or some other form of cooldown reduction, at this point you should be attacking around 2 times a second and maybe 3 times a second under highlander, and should be critting every three hits, so the only thing holding you back will be the frequency of your ability use. You can get a stinger for extra Attack Speed which may be excessive, but is also useful. Frozen heart can actually be a very useful item, even if it is for casters. It allows him to use more spells now that he has more mana and the armor makes him much less squishy. It also puts him well over the cooldown cap. Phage can also be taken to make Yi a little less squishy but to also allow him to stun his prey.
Master Yi already has an attack damage buff that is almost always active, so building attack damage wasn't my top priority, rather to make sure that he attacks fast. This is partially because it will greatly exaggerate the power of Highlander, but mostly because it allows for his Double strike to happen more. I also focus on critical hit chance because without it, Yi won't do enough damage, even with his unbelievably fast attack speed. I spec'ed offensively namely because talents in defense barely made Yi any less squishy, and Utility barely helps him at all. The one point given to Ability power in Offense was put there because I don't really like using exhaust, but if you are so inclined, you may put the point there and replace cleanse with exhaust. I gave Yi Cleanse and Ghost because Ghost enhances Yi's fleeing and chasing capabilities, and Cleanse can allow him to actually escape some ganks with minimal amounts of damage. Alternate spells that I recommend are Flash (A blink is always nice for escaping) Clarity (After a spree of using Highlander, your mana will be drained) or Exhaust (Giving him a damn stun.) Other than that, Yi is a whole barrel of fun if played well, I have had success with him, and can really be a ton of help. Good luck and have fun.