Hey, I play Master Yi a lot. I really like him as well, but no guarantees on my advice being good.
I recommend spectating some matches in the LoL client if you want to see how the best players are building him.
Generally, I want raw damage output for a couple of items and then a defense item. I generally top lane with him, if you jungle you gotta build different. So:
Doran's Blade
Berserker Greaves
Infinity Edge
Youmuu's Ghostblade
Spirit Visage
I sometimes get a Vampiric Scepter in the middle there if I'm hard pushing and mana is a concern. The defensive item would be different if I'm against AD guys and maybe even get it before the Youmuu's. SV is so good for Meditate, which I find to be the best skill Master Yi has. The Youmuu's allows me to blow up towers in a few seconds without burning my Ulti for it, and the value of its crit % is multiplied greatly by the Infinity Edge passive.
Attack Speed comes from ulti and Youmuu's active and boots. Since AS doesn't benefit Q at all, where crit % and AD do, I've found I gain more damage output from AD/crit. I also don't need waveclearing once my Q is level 3 or 4 so Statik Shiv is a waste. Sometimes for my 5th item I build a Ravenous Hyrda just to make it ridiculous, and then 6th is like Guardian's Angel or Randuin's Omen or Frozen Heart (depending on who you are fighting).
For runes I go flat AD reds, flat armor yellows, mr/lvl blues, AS quints. I like Teleport/Flash or Exhaust/Flash summoners, and a 21/0/9 Mastery setup. I tried 21/9/0 and other variations but I settled on the highest damage one with move speed and mana regen. I probably spam my spells too much and a better player would not need the mana regen, but extra move speed is real good.
Remember, Meditate is what makes you a troll hehe. I do R>Q>W>E and tbh I rarely bother using E's active component though I like to get 1 point in E at lvl 3 or 4 for the free AD. Since it has a specific cut off where losing the passive component makes you do overall less damage. It's like if the fight will last longer than 6 seconds you don't want to.