Wasn't at all challenging and wasn't at all worth $10 for each of the 3 DLC's.
Crown of the Sunken King had the best environment out of the three; the enemies were incredibly predictable and weak, the bosses were a disappointment, and it added little other content in the way of items and story. Hardest enemy in the game is Jester Thomas, an invading NPC. This dlc includes EASY PVE, low effort nashandra reincarnation that spams flame swath and summons velstadt, wannabe Kalamet dragon boss Sinh, flower petal skirt, and nothing else worth mentioning.
Crown of the Old Iron King was the shortest DLC of the bunch. EXTREMELY linear. Best part of the DLC was the Sir Alonne Boss battle which made me reminisce for Smough & Ornstein. This DLC includes a blue Smelter Demon, cheap NPC invasion that'll run away from you, Sir Alonne boss (very cool) and his armor set and a DiaKatana, and smelter 2.0.
Crown of the Ivory King was the better of the three DLC's adding the most in game items (armor, weapons, spells), bosses, pvp, and pve. It introduced a companion system that is very unappealing because typically summon help is unreliable and incompetent. The level design for the ivory king is better than the other two DLC's as previously unacceptable areas are unlocked as you work your way through your objectives.
Don't buy the season pass. If you're dead set on buying DLC, I encourage you to wait until FromSoftware releases an "All DLC included / Prepare to die" edition of the game.