Pros
Great Damage Output
FAST MOVEMENT
great farming
great for ganks
easy to survive and catch enemies
Cons
very susceptible to ccs
squishy
does not do area damage so limited in team fights

Include Dmg output runes so that you do not have to focus on that for you're build. Include crit chance runes for 100% crit chance by lvl 18. Other viable runes are health runes and flat dmg if you want easy early kills. I also include a speed rune because i like to be the fastest unit on the field but that is personal preference.

I usually go with this build cus the mallet gives you a lot of health. with the lifesteal, you shouldnt ever be in trouble in a 1 v 1, but if you are (like with trynd's ult. u can run w/ you're ult and come back to kill when his ult wears off). Start out with boots and hp pots. Follow that up with a vamp septer and upgrade your boots to berserkers. Then depending on the situation, you could rush phantom dancers (for crit build-dont worry about dmg output b/c of your runes-w/ this build u could get an executioner's calling for the crit chance and lifesteal till late game), bloodthursters for high damage and higher lifesteal, or mallet for health. This should be followed by an infinity edge. A second phantom dancer makes him fast and attack fast. At this point his move speed should be around 497. by the end you should have 456 move speed after you sell your boots to be replaced by a second bloodthirster. With the agility pot, you should have 100% crit by the end. Another viable build would be to replace the mallet with a trinity force (with this you would not need the crit chance runes and you would have very high dmg output with low hp-not very good against high cc teams but very good against teams w/o many ccs)

Cleanse and Flash.
Cleanse is great considering yi's only real weakness is ccs. Watch out though because this cant stop warwick's and malzahar's ult.
Flash is great for escapes and also makes yi the ultimate pursuer.

other viable abilities are ghost, teleport, and exhaust.
I've also seen ignite and rally on him but i dont think they are that effective on him.
Since this is not a guide for jungling you do not need smite.