As a Darius main, Darius is a strong pick right now. However, he's nowhere near as overpowered as people are claiming at the moment. If you respect his powerspikes and play around when he's the biggest threat to your team, I can assure you that a losing matchup to him will turn even or maybe possibly leaning in your favor depending on your experience.
Darius is a champion who's performance in lane will dictate how well he performs in a teamfight. This is true with a lot of champions, but especially true for Darius. It is important to know that unless Darius completely feeds in lane, he will always have presence in a team fight. Just because you killed him once or twice in lane doesn't mean you prevent his mid game powerspike. Darius is at his most powerful when no one is at full build and has the items to burst you down, meaning that taking extended encounters with him and allowing him to activate his passive will end up with him winning teamfights.
In the laning phase, champions such as Yorick, Jax, Ryze, Fiora, and Sett are all viable counters against him. Without his E, Darius can be kited extremely easy. If he uses his Q after you're slowed, make sure you get hit by the handle. Getting hit by the handle is worth the extra AA you might take from being closer to him as he WON'T be able to heal from it, and an AA does considerably less damage. Darius will get a big AD boost if he gets 5 bleed stacks on you, also activating his passive causing those 5 bleed stacks to apply to all other champions. This means that taking longer trades with him when you can't kill him with a single all-in will result in dying and losing the lane. The trick is to use SHORT trades. Do your damage, don't get hit by his skillshots, and get out. The person who gets the advantage first will most definitely snowball with it, which means you must not make mistakes in this lane. He will zone you from farm early and that's fine. It's not supposed to be an easy lane. You will both have your advantages and disadvantages. DON'T TRY TO CHALLENGE HIM LEVEL 3. HE HAS POSSIBLY THE STRONGEST ALL IN THE GAME, but after a couple of well done trades, things will turn the other way around and you'll get to manage the lane just fine. Whenever you need to step up to farm CS, you need to always ask yourself, "What does Darius have item wise and ability wise? Is he capable of chunking me out?" If Darius is using his W to farm, realize that this is a MAJOR component to him being able to reach 5 stacks on you. If he's using W to farm, immediately follow up with a trade as without his W, he really does not have any method of slowing you to proc his Q skillshot. Just one ability of his costs him 2 Hemorrhage stacks and the potential heal from his Q. This is an absolutely MASSIVE advantage when trading.
And for the love of god... If the Darius is competent, DO NOT ask for ganks post level 6. If your team has been oblivious to getting you a lead on lane up until this point, know that time has passed. Darius is most gankeable the first 6 levels. If you try ganking him afterwards, he will turn the fight around, apply max Hemorrhage to BOTH of you, and escape with 2 kills.
And unless you're a Tryndamere with full fury, NEVER just stand in his face and trade AA to AA. You Will LOSE. If you approach a good Darius and just trade AAs, he's going to:
AA - W - AA - E - AA - Q. You'll reach 5 stacks of Hemorrhage before you can even react. You NEED to KITE. KITE. KITE. If the Darius is bad, you can take some PTA Trundle Cheese and kill him if you both try trading AA to AA before he gets his passive up. This is a low elo cheesy way of killing him if he doesn't know how to kite properly
Again, Darius spikes HARD mid game when no one has the damage to burst him down. Even if he isn't fed. And quite frankly, this is how a champion should behave. You shouldn't need to be fed to have your presence heard as a threat in that game.
If all else fails, look to stall the game out as long as possible. Darius has a strong early phase, immensely powerful mid game, but a terribly weak late game.
But as long as he's the main priority in mid game team fights, he shouldn't be able to get his passive off easily. If you find yourself getting low on health, just back off out of his range. If you see yourself racking up Hemorrhage stacks, allow them to end before rejoining the team fight. CC works wonders against him. Even if he builds Sterak's, the tenacity he's given from that isn't enough to sustain chained CC. Darius HATES CC, as it prevents his rampage across the enemy team. Play safe the first few levels of the laning phase, make smart trades, and RESPECT his powerspikes as you would a Renekton. I guarantee you will see your performance against him rise.