Note that the only one of these I've played more than trying a little bit on free weeks is Diana, so I'm mostly going by what I've heard about these champs or my experiences playing against them. Other people might be able to give you better descriptions:
Fiora is a top-laner with lots of mobility and potentially huge damage, but she's generally considered a underpowered at the moment. I believe she has a very weak early game and low survivability without her ult, so it's hard for her to actually survive long enough to take advantage of her late-game damage.
Rengar is a jungler or top who's currently considered overpowered, but is getting nerfed next patch, so we don't know for sure where he'll stand after that. He's got a mix of decent defenses, good burst damage, good sustain, some CC, and his leap for initiating fights.
Diana is sort of a tanky assassin who usually goes mid or jungle or possibly top. She's got a ton of mobility for chasing enemies (though not much for escaping), great damage, and a shield. Personally, I find her a ton of fun to play.
Jayce is an extremely popular and powerful bruiser top at the moment. He can switch back and forth between ranged and melee. He's got great mobility with his gate and passive, very good damage and harass early game, and still solid damage and good utility later (since his gate is practically a built-in Shurelia's). A lot of champions, including a good number of popular top lanes, have a ton of trouble laning against him.
Darius has a reputation for being overpowered, but I think he's more of a pubstomper than anything else. He can be a huge bully in lane, and his ult is pretty ridiculous - it does a ton of true damage and the cooldown instantly resets if he gets the kill. He has no real mobility or escape, though, and players that know how to exploit his weaknesses can sometimes shut him down pretty easily, although he'll often completely destroy players who don't. His ult also makes him very good at getting tons of kills in a row, which might bother your teammates if they're the types to complain about KS but can be really satisfying and means he's relatively good at catching up from a bad early game. It might also worth noting that because of many people consider him an over-rated or overpowered KSing pubstomper, some people might just assume you're a noob and/or a jerk when you pick him.