I ran the darkness in kingsbane pre coldlight HoF. He was hard to get with 3 cards, when Valeera hp made 2 darknesses with 6 cards in the deck and you only needed to draw 3 for both of them, it won me a few games out of nowhere.
I have learned my lesson and only craft cards after the meta has settled a litle bit. Best example: I didn't immediately craft Shudderwock plus Grumble, Worldshaker after The Witchwood was released but waited. After about 2-3 weeks I crafted Baku the Mooneater to play Odd Paladin which was totally worth it.
Several decks ran Finja. He's hardly OP but definitely saw play.
I never actually thought, that they were competitive in any way. I just wanted to create fun moments with them and they felt like an amusing tech. However I have to admit, that I didn't play with them as much as I have planned.
Back in MSoG Cho'Gall was actually useful as a tempo enabler against all of those jade druids. He had nice synergies with the Kazakus potions as well, but sadly this was the only time I have ever used him. About Gromm - I don't want to offend any warrior players here, I'm NOT saying that Gromm is a terrible card to craft, but I was never a warrior player myself, I'm still missing a lot of class specific cards and I only played Gromm probably 2-3 times since I crafted him. This was back in KoFT right before the nerf to Fiery War Axe was introduced and Tempo Warrior seemed really interesting to me. Immediately after I crafted the deck, the devs decided to nerf the weapon and I never played Gromm on the ladder, so the dust is kinda wasted for me.
Imho she was kinda useful back in MSoG. I didn't craft her, but I got her from a pack. Immediately after that I used her in my control-ish Renolock and she worked really well against those annoying pirate warriors and aggro shamans in combination with Power Overwhelming. So imho this was the only real application for her, because the main issue was how to kill her ASAP and this spell fulfilled two different tasks - buffing & destroying.
Baku the Mooneater, easily. "but it's a good card, why you regret it?" I crafted by impulse. I don't have the Taunt Warrior kit, didn't had the Odd Pally kit (now I have but its so boring to play), and don't have the pair of Vilespine Slayers or Leeroy for Odd Rogue. Had I crafted Genn, I would be higher on the ranks today. Even after the nerfs (got it? huh? huh?).