Hi, let me just start off by saying that this... this my friend is my first champion build guide so please don't be harsh on me! As for this guide, well it's made reader friendly.

PROS

Great soloer throughout game - can take on two champions at once by hugging turret/ mediate
Good for cleaning up team fights
One of the best healing abilities in the game
Best Chaser/ Runner (highlander)
Good Turret assassin! (use highlander at turret can leave it half health when the other person has recalled)

Cons:

Squishy when not using mediate ( do not initiate team fights )
Always targeted first during team fights (which is why you should stay back and mediate when being attacked)
Prone to CCs

It's important to stress on cool down reduction because alpha strike, along with mediate and highlander rely on lower cool downs. This is why you should buy ionian boots of ludicity and deathfire grasp. (deathfire grasp is great also because you get to use it's active every 60 seconds and it does a huge deal of damage when the champ is full hp - 25% + 4% for every 100 ability power and yi has 500+ ability power - that's almost 50% hp)
When using Alpha strike - it's important that you use it on the closest minion or person and ALSO to make sure there are only 4 or LESS minions there. This is the key to harassing champions during early game. When they recall they you can use your highlander to snipe their turret out leaving you done with your task as midder and onto another task of ganking.

So remember... 4 or less minions and make sure the champion is near the minions as well, because what alphastrike does is it Makes a chain reaction. It hits the nearest 5 enemies and returns back to the enemy you used alphastrike on (which is why you want to select alphastrike on the nearest minion/enemy so you can return safe and sound)

This is a fairly simple ability - you just use it when you are low health. Usually I save it for when it is really low so I can maximize the amount of health I recover (sometimes meditate gives you more hp than you're missing) but this is very risky.
If you are being ganked - pull away and use this near a turret but if you're not near one use it after your enemies have exhausted all of their abilities that stop meditate - ie: Alistar's smash ability will make you leap in the air and thus stops meditate.

I love this ability because I am a very very impatient person. I usually try to save this for when I'm about to be ganked - sorta like escape rope from pokemon. However sometimes I use it when I see a place where I really really need to go - such as going back to my lane after recalling or helping my teammates out.
I also use this however to effectively wipe out a turret- but only if I know nobody else is around; because if there is someone around... well how the heck am I supposed to escape from that person if my highlander was used for the turret right?

Usually I just join team fights when the opponents have 1/2 health or less; that way I know that I'll at least kill one person. You should always target the champion with the lowest health so that way when you kill him all of your abilities get reset and thus you get to use alpha again on the next lowest hp and the cycle repeats. What's great about this is that you get to combine it with meditate when you're being ganked and then to escape being the area of focus, you use alphastrike (alphastrike gives you like a 1 second immunity because yi cannot be attacked/ targeted when he's zooming around).

When the enemy is all scattered after a teamfight - thats when you use highlander to chase them and kill them. Use your deathfire grasp and ignite to help you.