YMMV / Out of the Abyss

• Best Level Ever:
  • For part 1, Blingdenstone is a big favourite because of how much the fate of Blingdenstone depends on the player characters. The more they do, the more the town appreciates them, and the more resources they commit to the battle at the end of the town's mini-arc. It's a lot of work to get that Golden Ending, but it's worth every sidequest to become the Heroes of Blingdenstone, getting lots of magic items and gems, the admiration of the settlement, and potentially extra followers if the PC's return there during part 2. There's a REASON the book advises the DM to make it the climax of the first half of the adventure.
  • For part 2, it's the finale. Once all the demon lords have been gathered, the book strongly recommends dividing up control of them to the player characters to see which one is left standing for the Final Boss. You're going to have 2-3 at MINIMUM to play with, all their legendary abilities at your fingertips (minus lair actions, naturally), and no compunctions about collateral damage in terms of roleplaying. It's about as close as you can get to playing out a Godzilla film in D&D. And once there's only one remaining, it switches back to the players, who have to charge through the chaos and whatever lesser demons remain, in order to deal the Coup de Grâce to whatever demon lord it might be (the book presumes Demogorgon, and not without reason).
• Ensemble Dark Horse: Stool, the small myconid, has been rather popular among people who play the module. It helps that he's small and cute, and that he's genuinely helpful since he can use spores that have the same effect as the Comprehend Languages spell, making him a Translator Buddy for the party. A subreddit dedicated to the module has a ton of fan art of Stool, more than every other premade character combined.
• Memetic Mutation: Demogorgon can't swim.^note Players discovered that despite having a swimming speed, Demogorgon does not have water breathing. One would expect this to be a non-issue since he likely doesn't need water breathing, but it turns out that he still needs air to live. As he is introduced in the module bursting from the sea, players have joked that, if one was crazy enough to do so, they could find a way drown Demogorgon and end the module early. This has lead to jokes that the Demon Prince cannot swim or is afraid of swimming.
• Too Bleak, Stopped Caring: The initial segments of the module, escaping from the drow camp and cutting through Blibdoolpoolp to get to Blingdenstone, can feel extremely punishing, especially when closely following the gritty survival rules encouraged by the module. Particularly when the party's main goal is just to escape the Underdark, the oppressive ruination can be enough to cause a party to fall off before the actual plot begins.