Is this series perfect? No, and as someone who's read the wb novel I can say it is actually a massive story improvement writing wise to the point I desperately wish for a LN translation one of these days so I can see the steps of improvement the author has taken. With that being said I'll get down to the core of this lil review since a decent amount of people have said their bits on it. This review ain't getting numbers since I'm bad at that shit. There will be spoilers but nothing you can't figure out yourself honestly by first dozen chapters.
Overall rating: It's a fun read and I recommend going into it just to enjoy the characters.
In depth rating/explanation: This isn't the best series out there by far, but I don't think it's trying to be and that's fine. If you want something better, delicious in dungeon or Frieren are well known and beloved series at this place with near no flaws. Helck has an extremely similar tone of silliness but it follows it up with a heart wrenching emotional story that has earned it its rating. Go to any manga recommendation forum or reddit post, you will get a dozen series in the genre you want that will. What this series pulls off is being an entertaining light read that makes me happy to read it as of chapter 72. I like it the same way I like "The Mummy". Watching these idiots run aroun getting involved in stupid shit is entertaining and heals my soul.
Plot/story: I rate this a 'this is a VR video game manga what are you hoping for?' We follow our protagonist Kou who we discover can be an egg for both gender and species. His parents decide to deal with this problem by creating an in depth virtual game world and a body reconstruction pod where he can play as a half-vampire girl to get used to the fact that's what he's going to be in real life in 3 months, not telling him to fully allow his subconscious to make the choice of what he wants to be (this is the explanation given in the WN at least). Somehow making them both some of the shittiest but also in the top 10 for parents responding to figuring out their kid is trans.
If you're a fan of genderbend and most of its tropes there's plenty to enjoy from that side of things. As for the game and story itself, it's inside of a VR fantasy world with tech elements also in it. Seriously just read the first chapter the explanation of the world and its systems is mostly summed up there and it's all you need to know, this place isn't shotting for the moon. There is an over reaching plot but the author is slow to introduce it and I don't blame anyone who doesn't want to wait for the long time it'd take to get to more serious stuff getting involved. A lot of story beats are predictable enough you likely know where things are going at this point but they're not so blandly handled that it feels like a slog just to go through them, which is explained better later in this review.
Characters: I give this category a rating of 'Join the cult of Crim.' Honestly what makes this series along with its comedy. If the plot or story is a bit weak I can happily trudge through it if I'm enjoying the characters and that's exactly the situation here. Yachimoto really breaths life into this series with the expressions and artwork for the characters, and the many expressions people show in different comedic situations (especially Crim's) have gotten plenty of laughs out of me.
Like any feel good for fun story, the characters will either make it soar or will sink the entire thing because of the weight they need to carry, and the characters in DUO do that for the most part. There's weaker moments for sure and I'm expecting some cliche's to make me wince in the future. That doesn't detract from them handling the characters well for the most part and ensuring that they do the job this series needs of them: being entertaining to watch run around and do whatever stupid bullshit they got up to this time. Do they succeed? Yes. Every time they get involved in some plot point or story hook that doesn't really feel like it matters all too much to forming a proper main story, I'm still in it with gusto because it's the characters that end up making these moments interesting.
The author does still pull out some interesting surprises, and even then it still just highlights the shine these characters can have. Anyways there's a group of showy black bunny girl gyaru's so really this is what the last ~70 chapters have been worth going through for this.
Setting: I give this category 'yeah this would work for a pathfinder campaign or an actual MMO system' and will leave it at that because at the moment that's what you need to know. It's unique enough I remember the odd details about it but it's still not fleshed out yet at this moment to go further into it. Hints of greatness that we can't tell are actually greatness yet or just little plot holes of the future.
Final word: Look I've said a lot of words here but I'll say the one thing that me and my friends who have read it all have it boil down to. DUO is a fun read. Maybe for you you need something more, and you'll likely find that on mangadex in the 9+ star categories. But for me DUO being just a fun story to read is enough and for the people I introduced it to it was for them too. Come into this with that attitude and you'll be able to just appreciate the silly moments, fun faces, and comfy vibes with occassionally cool fight scenes that crop up. Maybe one day it's all going to devolve into slop like other stories have by trying to be too serious at one point and losing the silly clown paint polish that has it shine, or a thousand other ways it could fail and falter. Until that day though, I'm going to keep faithfully reading with excitement and hope that the next chapter is going to make me either laugh or get a new reaction image face just like all the others.
As an end note, thank you Church of Crim for your translation work up until this point. You're all a blessing and have given me the joy of getting to read this series. Whatever anyone else might say, I appreciate your dedication and am forever thankful. Cheers.