YMMV / Fiend Folio

• Best Boss Ever: Warp Zone is easily the most popular boss in the mod, due to its fun Bullet Hell patterns, using bosses from earlier floors in some phases, and, of course, that iconic Super Meat Boy "WAAAAAAARP ZOOOOOOOOOOONE!" scream heard on the versus screen.
• Breather Level: Compared to the nightmarishly-hard Dad's Home+ challenge and the moderately difficult Dirty Bubble challenge, the Real Jon challenge's only gimmick is a giant, black Death's Head slowly bouncing off walls for the entirety of the run. While this presents a few problems (chiefly, the head being able to block shots), the challenge is otherwise identical to a typical Fiend Folio run, minus Isaac starting with a D6.
• Crosses the Line Twice: Ghostse. A ghost that serves as a reference to the infamous Goatse shock website, with the ghost firing poop shots out of an anus-like opening in its chest. Its variant, Ripcord, isn't much better, being a specter who not only has its design and animations inspired by common depictions of flashers, but it even shoots out a string of flies based on anal beads for its attack. Mind you, it attacks Isaac, a child who is only five years old. No wonder it was developed and added to the game in secret against the wishes of the mod's other developers.
• Fridge Horror: As detailed in the Nightmare Fuel page, the Starving is a Fatty variant that eats other enemies, and then vomits their chewed up remains as an attack. The only exception is the Incisors, which spawn Leper Flesh that they eat instead. Let that sink in...
• Game-Breaker:
  • The Dazzling Slot spends nickels to instantly turn enemies into Golden Slot Machine payouts at no extra cost and with no cooldown; save up and you'll never worry about health or pickups in regular rooms ever again. The only drawback is that it won't appear until you kill Mega Satan as Tainted Fiend.
  • Rat Poison is an active item that allows you to effectively skip all rooms by a certain author for the remainder of a run. Already powerful enough as-is, but when used on either prolific enough level designers or on certain level designers who specialize in harder rooms, it effectively turns the remainder of the run into a cakewalk, with the only downside being that it can clear special rooms as well. It is as busted as it sounds, and naturally has a lower weight of appearing in item pools.
  • Birthday Gift replaces all subsequent items found with Mystery Gift, except for set drops, rerolls, and items dropped from Mystery Gift itself. Since Mystery Gift's rewards draw from the pool of the room it is used in, this can easily be abused to dump Treasure Room and Boss Room items in the Secret Room to search through that item pool, to bring an extra item in to an Angel Room or Devil Room, and so on. It can also be used to bring an item over to a reroll machine or group items together to reroll them all at once. It is essentially a far more controlled form of Chaos.
• Genius Bonus: Maybe even more so than vanilla Isaac.
• Hilarious in Hindsight: The very end of the trailer for Reheated's release shows a missile gif blowing up Fiend and Golem, come 1.7.9 for Vanilla Isaac, and that same gif is now featured prominently in the G-Fuel Daily Challenge.
• Memetic Mutation: Pornograp ^ExplanationA cropped picture of a red Isaac with a smug expression with the above text (the uncropped version reads "This item is a reference to Pornography") has recently went viral as a reaction image, especially on r/bindingofisaac
• Nightmare Fuel: Has its own page. Being a Denser and Wackier version of a horror game doesn't mean there's no horror, after all.
• Nightmare Retardant: Like everything in Fiend Folio, Cacophobia isn't totally devoid of humor, because sometimes Venus can randomly become... a bust of none other than Venus de Milo, with nothing particularly frightening about her aside from her surroundings.
• That One Sidequest: "Dad's Home+" is widely considered the mod's hardest challenge. The main mechanic behind it is that every enemy and boss encountered will respawn when entering a different room, including rooms that have been cleared out, and need to be killed again to move on. Many of the new enemies added to Fiend Folio are harder to manage than vanilla ones in their own rooms, let alone when added with others in rooms not designed for them. This will often lead to being overwhelmed by enemies and taking virtually unavoidable damage. Because of enemies appearing in every room, going to Treasure Rooms is discouraged unless one is right next to the start. Overall, the challenge is massively luck-based and involves going through a massive Zerg Rush at near-base stats. It's telling that the player is given a free Treasure Map and Compass to find the Boss Rooms immediately and the goal is just Basement/Cellar/Burning Basement II's boss, yet this challenge is still infamous for its difficulty.