Yesterday, I finished the game with Alywyll, my Dark Urge character who's not... completely evil but not worried about being good either. For her run, I usually went with the darker/more selfish choices available, so yeah, it would be generally considered an evil run. And I have... opinions on it. First, for the non-spoiler overview:
I haven't played many "evil paths" in RPG games (mostly because I hardly ever replay games; BG3 is proving an enormous exception!), but I read people saying how, in many cases, the evil path isn't so well-written. BG3 has a different issue: it is well-written, but it's lacking in content compared to the good/neutral run.
The Dark Urge storyline itself is great. I've said it here before and will say it again: it's worth playing a Durge evil run because of the unique interactions and powers you get. The "second gift" you earn by embracing your Urge is one of the coolest things in the entire game!
But, apart from that, there's nothing "unique" to the evil run (especially when you consider you can embrace your parasite's potential in either). Siding against the Grove in Act 1, for example, will lock you away from three, potentially four companions. That's four quest lines you'll be missing out on. You'll also lose Dammon, the blacksmith, who sells some great equipment in Act 3, along with all the other Tieflings and their quests. Honestly, I don't have a big issue with the morality of siding against the Grove; the issue is how much content you lose and gain nothing... I mean, you gain Minthara and (if you choose to, but why would you not?) the best sex scene in the game, but Minthara herself can only be recruited in Act 2, and while I love her to pieces, she doesn't have much content either after you decide her fate in the second act.
The gist of it is: the Dark Urge doesn't need to be evil (you can, I believe, play it three ways: good - resisting the Urges -, bad - embracing the Urges -, and a mix - which is Alywyll's case), but if you'll play an evil run, do it with the Dark Urge. For now, there's no reason to go evil with a "normal" character. I hope that Larian will add more content in the future; perhaps some quests involving Goblins, and pleeeease more Minthara goodness.
Two Dark Urge tips: The Durge has a fantastic romantic relationship with Minthara, so worth it! Astarion is also essential on your team! His high-approval greetings (you'll easily get approval with him on an evil run), especially after you discover the truth about the Durge, are the best! xD Example of a non-spoilery one:

About the ending. I wasn't a fan of the evil ending either. To explain why, though, I'll need to speak in the forbidden language of SPOILERS:
So, for the evil ending, rather than destroying the brain, you take control of it. Great! The problem is that your other companions become thralls. That was decidedly not what I wanted or what the game promised me. As I developed my relationship with Minthara, the plan was clear: take control of the Absolute so that we could rule it together. That the game offered me no option to rule alongside her and my other companions feels very unrewarding. And then, while you get 10 minutes of cutscene plus the epilogue with the good ending, the evil ending closes with a short cutscene of you and your thralls atop the Elder Brain.
Unless you're roleplaying as a character who literally cares about none other than themselves, choosing to destroy the brain (I went back and chose that. Or, better, I proposed the Emperor for us to take control of it, he didn't like the idea after Alywyll failed a charisma check - I reloaded the save until she succeeded and he just took control for himself and turned us all into thralls -, and then destroyed the brain) is, in my opinion, more fulfilling. Minthara has literally no line on the beach after destroying the brain, but later you have a scene where you can choose to take control of Baldur's Gate with her. That is a good ending! She's extremely funny in the epilogue too. Minthara, thanks to the awful society she grew up in, only sees parties as a chance to poison others. She says this, and there's an option where you can say something along the lines of, "You grab the drinks, I'll grab the food. They'll be dead by morning." I stared at the screen for a whole minute straight trying to figure out if we were joking or if we'd actually kill the few companions we had left..... thankfully, it was all in jest and no one died (unless it happened after Withers' speech, but I like to think we were too busy then to care about the others) 😃
I'm also not sure how to feel about Lae'zel's arc. Perhaps it was my fault for ignoring her for most of Act 3, but since I blew the créche up and she never learned the truth about Vlaakith, she was loyal to the end. Literally to the end scene at the beach, where she was like, "You know what? I don't need ascension. I'll spill blood on my own terms. Fuck Vlaakith!"
Overall, I had a lot of fun with the Dark Urge and would love to play it again fully embracing the Urges. I ended up refusing Bhaal because someone told me the Durge would kill everyone in the epilogue if we accepted to be Bhaal's Chosen, but for Alywyll, I honestly prefer the refusal arc. It suits her better. (One change I'd have done in this playthrough is stopping Shadowheart from becoming a Dark Justiciar, but since I didn't know the truth behind the Durge in Act 2, I let her kill Dame Aylin.)