So you think Yi is freelo, no skill, and ez? Well, you're right, sort of.
These assumptions probably stem from you being absolutely annihilated by one.
However, there is more to Master Yi than just autoing away and getting Pentas.
In this guide, you will learn how to play Yi every way possible and learn the mechanics behind every good Yi player. Good luck!

Pros
+Very good scaling
+Relatively easy jungle
+Double strike works well with Guinsoo and Devourer
+W Flat damage reduction very good
+Q can pretty much dodge anything
+Resets=Good
+True damage on EVERY auto when E is active
+Autos lower Q cooldown by 1 and you have a lot of attack speed

Cons
-Q can sometimes bug out... "But I was in Alpha!" -Cowsep
-Doesn't have any CC at all
-Relies on fully sated Devourer to do anything
-Fear is really bad for you while ulting
-Little to no ganking potential

Double Strike: This passive compliments everything in Yi's skillset and build. Very useful overall, nothing much to say.

Alpha Strike: Very important spell to keep track of. This spell is half of Yi's "mechanics." First off, Q is your pathway to making plays with Yi. Alpha strike allows you to dodge ANYTHING, or go across ANY terrain. While in Q, you are essentially not in the game, you become untargetable. What's even better? Q's cooldown goes down 1 second per auto and resets partially when you get a kill/assist. Always look for times where you can use Q to dodge skillshots, traps, and incoming projectiles, or any other spell.

Meditate: The other half of Yi's mechanics. This ability reduces ALL incoming damage by 50%, which means you can time this to survive or reduce high damage abilities' power. You can also use this spell as an auto attack reset which will help you duel better, reset your Q, and proc more on-hit effects. To reset your auto: you need to cast W DIRECTLY AFTER your auto finishes.

Wuju Style: Another ability to compliment everything else. True damage on every auto, and Yi attacks a lot of times. Nuff said.

Highlander:

-1/25/2016: Guide Created
-1/28/2016: Worked on Abilities
-2/6/2016: Changed "Guinsoo" to "Gwinsoo." Credits to N3WB13P13 (Jonathan Zhuo)