DPS or AP? This question has been asked for a long, long time. Its decent if you have a dps Yi on your team, but if they stack armor. you could just lose the edge there. So, im going to introduce a new style of yi here.

Guinsoo's Rageblade stacks a bit of AP, as well as attack speed as well, making it a good item to have early game.
Berserker's Greaves is a nice item that gives Enhanced Movement Speed 2 and gives a attack speed bonus.
From here, you should probably buy a Hextech Gunblade. It grants 1:Life steal 2:Spell Vamp
What more would you like? Nice decent item for a hybrid build.
After the Gunblade, you can take a Trinity Force. I usually build a Sheen->Phage->Zeal for Ap->Hp->Attack speed. Trinity force is a good overall item for Yi, because it gives a lot of extra stats
Now onward, if the game hasnt ended, you buy Infinity Edge or a Rabadon's Deathcap. If you like AP, go ahead and take a Deathcap, but I normally purchase the Infinity Edge first just for Damage.

Take a vampiric scepter for a first item. It normally lets you stay in lane for about 2500~ gold. Then, port back to base and buy a Guinsoo's Rageblade. Then, you should probably settle for Boots for the Movement Speed. Then a Hextech. Focus on the Cutlass if you have money.
After the Gunblade, Trinity force. It is a all round item. Then the two items. Game shouldve ended already ._____.

Im not going to comment on the Masteries or Runes. You can play without them, and still get a decent score. Its Personal Prefrence there.
I take Ghost and Cleanse. Stun is a very heavy factor for Yi. If Veigar or Other CC Champ pops a Stun, press F. GTFO.
Ghost is for getting away, or just in synergy with Highlander. It helps in a lot of ways~

Yes, I admit, I keep on repeating things :P Sorry for inconviences
Mkay, nothing more here.
Comment and Rate please :D
Angel, /out.