So, i'm abstaining from playing/buying darius for a couple of days so everyone can get him out of their system and i can have my turn with him. Played with/against in a couple of ranked smurf games, a couple of sentences on darius:
1. The longest range of any of his abilities is Apprehend (e) with 550 range (for those of you who are unfamiliar with unit range, 550 is the average auto attack range of the majority of ranged heroes, be they casters/ad). Thus, Darius's range is somewhat 'low' compared to bruisers/assassins with higher blinks/dashes (Irelia, Talon, Jax, Jarvan).
2. Darius' damage is quite high within his zone of control (Max rank Q sweetspot does 315 physical base with 1:1.05 bonus AD, meaning many can and will get early AD into DEF to abuse this) and the ability to stack his passive to full over a short period of time.
because of this, when fighting darius (as a mage or bruiser) i see only two viable strategies: Outdamage him within his zone of control [Garen, Lee sin, Riven] or poke him with spell combos/assaults with frontal burst (Jarvan IV's E-Q-aa, Jax's E-Q, etc). Attempting to skirt his range for last hits or try to deal sustained damage with heroes such as Tryndamere and Gangplank will leave you wide open to eating an Apprehend into a full spell combo, which post level 6 leaves yourself open to an ignite + Guillotine followup for a kill.
For the reasons stated in 1 and 2, i think Darius is more suited to pack Ghost than Flash, as even at early levels a 5 stack of hemorrhage will deal significant damage and can lead to an early kill combined with W spam with a high bleed count. (When considering Ghost vs Flash in Draft modes, always evaluate the enemy team. Don't take ghost vs heroes like jarvan who can lock you down easily.)
His early game damage will be quite weak if you buy armor, especially so if you can spec into Indomitable in defense, reducing damage by 2. This decreases the amount every DoT tick will do to you by a flat 2, which means a combination of this trait + armor leaves a low bleed count early game negligible.
I'll probably write a post up after i've had some games and spent some lab time working on an awesome build, but he is a hybrid melee carry and a hybrid AD caster. Earlygame, he relies on spell/bleed damage to provide him with an advantage/zoning potential, but endgame his scaling on Q means its main usage is to apply hemorrhage on as many targets as possible for movespeed bonus and focus on abusing high damage auto attacks and W spam (200% damage on a 3s cd? T_T) ala tryndamere. A mixture of the two builds (with some beef!) should generally net you good results.