Ap Yi is fun bc he has so much survive-ability, that you never have to leave a lane once you have your first 2 items.
Ap Yi can also do good turret dmg if you pop your ult.
Your Alpha strike is amazing for farming, and can do great burst dmg. it is not ideal in head on fights bc even once you have your first 2 items its cool down is still 7 sec, but it is very good at poking, her***ing, and picking off low health champs.

Ad yi is fairly simple, at full build he is retartedly good at bursting down champs. at early game he is only realy good at ganking.
He does well more than 800 a hit, and he always hits crits.

Yi is not good in team fights if you build him Ad, bc he can only atk one persone at a time and to my history i always get focused in team fights as Ad Yi. Ad Yi is rather glassy so he dies in a second if focused in team fights.
Ap Yi is not very good head to head, but he is great in team fights. He can take all the dmg if he gets focused and he can hit every champ in a team fight with great dmg every 7 sec.