I must admit that I'm quite torn between "miserable, but liking it" and "miserable, but can be fixed with a few tweaks". For my Tav (Orphic Githyanki necromancer), the ending she got surely felt horrible.
Sava had hoped that either the saved Gondians or Dammon with the enriched infernal iron could fix Karlach's heart. Both Sava and I thought Wyll completely lost it when he announced his rebranding, and I still feel the Wyll we meet at Reunion Camp rather spent a few weeks lost in the forest with his "pet wolf" Scratch trying to save his new 1st-level ranger self from a bunch of goblins and an ogre, than in actual Avernus. So, basically Sava was forced to see her ra'stil and friend turn ghaik. From her vantage point, Lae'zel had an egg to hatch and a revolution to fight along with Orpheus, and she sure wouldn't turn herself. Karlach said she was happy, so that was the ending of my playthrough. For Sava, it was a tainted success, for a path of recruiting a saved Omeluum, killing the treacherous Emperor on the spot and saving Lae'zel, Karlach and Orpheus seemed in reach. I guess I'm wondering if this didn't work due to lack of story development, or if it just wasn't in the cards.
