my problem with ap yi is that i've NEVER seen a good one, never ever

Dump him in a lane wiht a caster who has no resistances who insists they can dodge it/stun him before it happened.

Yes, it was frustrating watch Veigar feed the Yi. who then proceeded to nom on the rest of us due to 3-4 early game kills :|

Nashor's on AD Yi is something I don't see the value in!

It's quite useful, the CDR/Mp5 alone is excellent for him. The AS lets you proc his passive more, and the thing to remember is often times as AP Yi you'll need just a bit of damage to finish someone when your Alpha Strike is down. Also remember he has Wuju/Highlander so with just a Nashors you're gonna be hitting incredibly fast during your ult and with a decent bit of damage against squishies.

EDIT: Silly me, thought you said AP Yi. Either way my post still works.

If you want to att hard and fast I suggest...

(Btw, this is an AD/AS/Crit build)
Phantoms
Zerker boots (Merc's if they have a lot of CC)
Youmu's Ghostblade
Infinity Edge
Black Cleaver/Starks/Bloodthirster (This item depends on team comp)
Situational item (Defensive or Offensive)

Defensive Options(typically):
Guardian's Angel
Banshee's Veil

Offensive:
Bloodthirster
Last Whisper
Executioner's Culling (if no ignite on team)

EDIT: I don't get 2 Phantom dancers because I think the crit and AS from youmu's, 1 PD, the IE and your Highlander is plenty of damage. The elixirs (especially red and green) help out greatly too. I'm not a big fan of pure glass cannon as a melee character.

Feel free to ask if you want me to elaborate of a specific item choice.

you can semi-tank in meditate with enough ap. you can build rylais for health and an AoE-ish slow. plus the passive of highlander means that you activate it, alpha strike for a kill on a low hp opponent, which refreshes his cooldowns and allows him to chain a 1:1 ap ratio AoE-ish nuke. its not a bad build when played correctly.

nice build guys but i think mine is also good ( mid-late game)^^
got 34 procent crit damege runes and flat MR and flat armor
masteries 21/0/9
first berzekers or mercury ( depends on enemy team)
then phantom ( need to get the crit chance fast)
then i usually buy a vampire scepter for flat lifesteal then i safe money for infity edge
thhen youmoo for ultra bd
the rest depends on my game and the other team ^^

lol i also have divine in my name ^^

You guys are cute.

Personally, I've become a huge fan of Gunblade as his first item (I just love it in general, I've learned....), because it gives you so much flexibility and potency.

AP Yi, like many champs, is entirely team dependent. In order to perform to your fullest in 5v5 encounters, you need a team with a lot of AOE or multi-target effects who can drop multiple people 'low' for you so that when you jump in you're fairly sure to get a kill, and then unzip their team with a barrage of Alphas (a 'barrage' of 'first strikes', mhmmm).

Either that, or you need a couple hardcore bursters, and you have to launch into the team before they unload. You tag most of the team with your Alpha, Weegar or Annie or whoever come in to nuke someone to bits, you get your first reset and you Alpha again.

Alpha Strike is a 4-target 1:1 ratio. With Highlander, it's easy to have hit a total of 10 times over a fight with it. Theoretically more are possible. Your output is 1000% of your AP. If Annie hits EVERY champ with all her spells, including two disintegrates, (A lot harder since two of her spells doesn't auto-target like Yi, who can hit loads with it's range) - she does 865% of her AP. Yi offically hits harder than Annie overall if you can actually proc highlander. If you can't take down ANY targets, so highlander never works, he's pretty useless, but if you proc it even once he can easily take a teamfight pretty much by himself.

He's also very mobile for a caster, hurts turrets more than most casters, and has more sustain than most casters. Just remember to get a Lichbane on him.