I usually classify Xin's item paths depending on how they're built:
Warriors/Youmuu's - this build focuses on very early game, rushing for double brutalizers (Warrior then Brut) gives him very high amounts of AD and armor penetration early on. This means his early game damage becomes absolutely ridiculous once you land all 3 Q's, since you can get 2 or 3 more auto's off with good movement. However, this build is meant for early assassinations only since you don't have any high sustain or tank mechanism that allows you to hold onto chaotic, 5v5 teamfights. Your dueling potential also falls off since your stats are mainly flat penetration and AD before building tank, but you do stick fairly well due to the movement speed from Youmuu's active once its completed.
Warrior/BotRK - I feel like this build isn't exactly the most optimal. Unlike the previous build, you gain dueling potential rather then assassination potential since the lifesteal now stacks with your W to give better sustain potential, while BotRK allows you to scale somewhat into the late game with its % shred. That said, I don't feel like it allows you to stick very well onto anyone since all your slows are fairly quick in duration - your E, possible Randuins, and BotRK aren't all very long in duration, but you can peel fairly well by taking out tanks with your innate 15% shred and armor penetration, so it's a fairly brawling build.
Devourer/BotRK - you can scale. That's kind of it. Provided you can farm stacks either by killing the enemy team or by farming the jungle safely, 40 minutes into the game nobody can realistically duel you. That being said, neither items are good early game, so you're basically compensating for him falling off lategame by building him for late, but sacrificing the multipliers you can potentially get early on (Devourer, the instant you get it, is nowhere close to as powerful as Warrior could be on Xin). I prefer this build the most in solo queue since I really like attack speed on Xin.
Devourer/Hydra - this build is 99% a farming build. You do have burst from Hydra activation and its 75 AD but its offset by the fact that Devourer needs time to scale up. Really strange build for a 2 damage item combo and not something I'd recommend.
Warrior/Black Cleaver - I'm a bit on the fence about this one, but its very good for peeling in my opinion. 15% shred + 25% shred, given your ADC also has last whisper, absolutely wrecks any enemy frontlines' armor. Functionally similar to the second build, but your main focus is on peeling rather then dueling or diving in my opinion (ADC's don't build so much armor that you need those forms of multiplier even if they're good).
Personal - I personally run a 3 damage item Xin Zhao if I'm ahead - Devourer into BotRK, with Hydra as a 5th item. This gives me ridiculous amounts of sustain, if I get a GA and/or a Spirit Visage I will duel anyone and win. I get Hydra as a 5th item since that's the point where I decide if I'm ahead enough to warrant it - this gives me time to analyze the situation to determine if I need to go tanky, if I need to build something for the team (Righteous Glory, Aegis), or if I can go assassin Xin. Very very risky since your midgame kind of falls off when your Devourer isn't stacked to the point of contention and you don't have any realistic tanking ability except for some lifesteal.
I like maxing E. Maxing gives you far more options - you gank better due to a stronger slow, lower CD, and higher instantaneous burst damage. It's also, in specialized situations, an escape if you can target a creep or a monster in the jungle over a wall. And finally, never underestimate its AoE damage - it actually clears somewhat faster then Q maxing, meaning you have the option to gank more at relatively higher health. Finally, its magic damage - people expect Xin to be physical damage so the burst of magic damage early game tends to surprise them more because, unless your laner is magic damage, the enemy laner might not necessarily take MR runes.