Originally Posted by Leirkov
GO AP. hurrhurr
But to answer your question, a mix of AD/AS is good, but he needs to be able to dish out a truckload of damage, so a combination of items like Infinity Edge and Last Whisper are good. Remember that Yi has a built in attack speed increase in Highlander, so he doesn't need to stack a ton of AS items.
also get ghostblade because its basically a second ult without movement speed. my yi build is as follows ghostblade, phantom dancer, bloodthirster/starks fevor, infinity edge, and if there are dps chars thornmail. and berserker greaves