Q. Attack speed Darius... what the hell?

Yes, Darius is usually played as an AD bruiser -- high health and high attack damage. AD Bruiser Darius has his uses, but if your team already has enough health for group fights (or the enemy team is particularly squishy), then Darius can be converted to a powerful Assassin/AD Carry. He loses some value in 5-vs-5 group fights, but becomes a devastating force in duels or 2-vs-2 engagements. Nasty ganks.

Don't worry, his ult will still do great in group fights, especially for cleaning up the survivors. Unless your team has plenty of chase or it's the end game, don't be the engage if you think it's going to get you killed. Even if the team comes out on top, you're more useful alive to pick off the stragglers.

AS Darius maximizes DPS while laying stacks of Hemorrhage on the target as quickly as possible. At 5 (max) stacks, Darius' Crippling Strike has a cooldown of 3 seconds (this is reduced even further by the cooldown reduction from his items). More importantly, stacks of hemorrhage increase the true damage from the ult.

Q. Wit's end... what the hell?

A. Wit's End may seem like a curious choice, but its passive provides a huge on-hit damage boost for its gold cost. As an early item, it's invaluable. The attack speed helps stack hemorrhages, and (because the damage dealt is magic damage) Wit's End devalues any armor the enemies in your lane might have rushed to deal with your physical damage. If you find yourself in the late game without a wit's end (or extra gold), feel free to disregard it (or sell it) and purchase a heavier AD item.

Q. Any tips on ability usage?

A. AS Darius can sometimes take more finess than AD Bruiser Darius. He has relatively low health and hemorrhage caps at 5 stacks (which you'll hit quickly). Most importantly, you'll have a speed advantage due to the zephyr. This means strategically disengaging from a melee opponent while waiting for cooldowns to recharge can save you some health (and bait them in to using their gap close/escape too early, if applicable), something AD Darius would rarely consider.

In general, save your pull for when it counts. You're much scarier when your pull is off cooldown, and pulling back an enemy as they flee will almost always result in a kill.

If not soloing a lane, take a lane partner with crowd control to keep the opponent from running until you've grabbed Furor enchantment or Frozen Mallet. AS Darius is a bully, and they WILL be running from you.