I'll do a quick assessment of Dark Knight and Lady Luck since I'm flailing on updates and now that I have a new PS3 controller, DMC4 is likely going to be my main thing for awhile.
Dark Knight - Easily my favorite class in this game so far. Part of the reason is that it was the first time the class ever showed up in a playable fashion since FFIV. I mean now, the class is everywhere in the series, but at the time, it was the first time the class had been a player controlled in nearly a decade. The main draw of this class is its tankiness combined with its sheer power. The class has the best natural defenses of any of the classes I'm currently using and it's attack power feels slightly higher than the others. It's innate skills it can learn gives status protection from many of the worst status effects meaning this class is great to combo with an accessory or Garment Grid that offers healing abilites. Hell even the magic stat of this class is no slouch though hardly ideal. Darkness is also just an excellent skill that is great for trash mobs and good against bosses as well since it's non-elemental. The HP cost is fairly low and its also bugged so that 0 Mp cost abilities will negate the HP cost completely making this class better. It's basic attack is also strong and it comes with some useful spells for dealing with enemies and keeping the DK alive. Charon is a throwaway skill, but Black Sky is a pretty good ability and I love the fact it's basically the Meteor spell. It's biggest drawback is how slow it is, but I feel that can be mitigated by the team composition. I never noticed the class being slow while using a team of Alchemist (also a slow class) and Lady Luck (can often muli-attack depending on how good your reels are) whereas I can see teaming it up with a Berserker or Thief would make it painfully obvious how slow the class is. Still, the class beats out these other classes due to it's high survivability and innate protection from several status spells. This is something I've noticed to be a problem with this game as it seems like the combat and classes are really based on a YOLO concept with even traditionally beefy classes like Warrior and Berserker often feeling flimsier than necessary. I'm not surprised this class is often a mainstay for endgame bosses, it really is in a league of its own so far.
Lady Luck - A fun class built on high risk and high reward. Easily it's best qualities is access to it's innate skills which double XP, Items, and Gil as well as making all attacks critical hits. The Dice moves are fairly broken like they were in FFVI, and much like Wakka's Overdrive, the reels are devastating when you get them to line up just right. I do find the item reel to be a bit pointless but the other three are pretty awesome with the melee reels giving you some nice physical attacks with debuffing properties, Magic gives you access to Black Sky and freaking Ultima, while the Random reels can go everywhere from having them cast a basic cure spell on the party to an auto-win move that doubles all your rewards in addition to just winning the fight for you. Granted, Dud is a serious drawback as it will remove 75% of the party's health when activated, but considering the reels are susceptible to the same cheat you can use for Setzer's reels, you can generally avoid this. The class also teaches a skill to raise Luck (crits) and one that can Confuse a whole group of enemies at no MP cost. You also can get Bribe, but I've never found that skill useful outside of item farming late game crap in FFX. I would say the only true drawback of the class is it's low health and defense. It hits surprisingly hard for a class using playing cards, but it's HP and defense are comparable to the Alchemist and pales in comparison to beefier classes like Warrior and Dark Knight. Reels can also be a pain for people since using them without the exploit is dangerous while using the exploit is going to offset the rhythm of battle since you'll be going all MM1 on it. The class also might be the worst dress. I mean they look like cocktail dresses that got fed through a wood chipper. Only Pannacotta Fugo would say their outfits are magnifico!