What was Riot thinking? This champion is not balanced, not a good addition to the game, and not fun for anyone other than the player currently using him.
He's counterable in the same sense Tryndamere is. But he's far, far more threatening than Tryndamere.
Anyway, I don't have much else to add. This is bad game design, and I am saying that from the perspective of someone who's been playing him exclusively for several days (rather than someone who's been getting killed by him for several days).
I'll expand a bit: He is very strong against most melee champions. Having zero opportunity to outplay and go against a fed Darius is frustrating when you're playing against them.
His mechanics encourage not just taking kills, but taking multiple kills quickly even if allies could have gotten them. Playing w/ a Darius on your team deprives you of some enjoyment, but that's not the fault of the Darius player so much as the champion design that encourages the Darius player to take as many kills as possible. Even built heavily for tanking, he will perform wonderfully.
And in those situations where you shut down a Darius with crowd controls or just overpowering melee range damage, the Darius player themselves has no counterplay. This is because he's an all-in champion with no gap closers, no speed boosts, and no escapes.
In the end, either Darius is not having fun, or up to 9 other players in the match aren't having as much fun as they should be. The fact that he can -- and is encouraged by design to -- actively make the game less enjoyable for allies (especially AP/Ranged AD carries) just isn't good design.