Black cleaver on Darius?

First and formost the item gives him stats that he uses well. It gives a nice amount of raw AD, and Darius's abilities scale well with AD. Plus, his passive bleed dmg deals Magic Damage that scales off of his Attack Damage stat. So building AD makes your auto's hurt more, makes your q and w hurt more, and causes your bleed to deal additional magic damage. While any item that gives AD does this, Hydra gives alot of AD being outside of a BF Sword built item. It gives Darius outright lifesteal, which helps him sustain, plus it gives him HP Regen as well. Lifesteal applies to the AoE on passive attacks and the Hydra's active.

So it gives him ALOT of hidden sustain while simutaneously giving him more dmg. And yes, it makes every auto attack a mini Q. You basically Auto attack, q, auto attack, and you've nearly wiped the entire minion wave out. Stack on your bleed dmg passive and you should have no trouble picking up the gold and xp on every minion in your wave. And yes the Hydra active does reset his auto's, which means not only do you get the Active's nuke dmg, you sneak in a faster auto thus an extra stack of bleed.

It also helps Darius smash towers. Auto, w, autohydraauto, repeat until w comes off cd. Darius does nasty things to towers left alone to do so. The bonus dmg on his w IS applied to turrets in addition to resetting his auto attacks. If things go well the Hydra can be your only AD item you need, outside of building a Maw of Malmorthius or a BotRK if you have something rediculous like a Volibear, Cho gath, or Rammus on the enemy team (active/passive)

It's an item to me that has a place in every game for Darius, wether he's going full ad, full tank, or bruiser. He makes use of every stat it provides, nothing is wasted. For me, it's a stable item for him. Exceptions on NOT rushing it would be for example vs a Riven. I recently fought one. Started Doran Sheild, Cloth Armor ~ Chain Vest = At this point Riven's trades against me did negligible damage, and I would apprehend her with e each time she tried to back away to further punish her.

When ever her Naut showed up to gank for her, I'd just target and kill him first and turn back on Riven when she comes in trying to collapse. Outside of very few situations you really should buy a Hydra on Darius.