Hundreds of other reviews go into detail about the excellent worldbuilding and gripping story - I won't repeat the obvious. Best on this site, bar none. What I will say is that the humour is genuinely great; from very first dramatic choice on the train in the opening minutes and the Dark Souls reference once you arrive, it just keeps going and going. I laughed more than I care to admit - my only disappointment is that I can't just share all the awesome moments with everyone I know, because they come from an adult game.
I think what impressed me most, though, is the production quality. Aside from the insanity of pulling all this off with Honey Select 2, Caribdis goes above and beyond in ways few other VNs bother with. The facial animations are amazing on their own, but the way editing, quick alternating cuts, and details as simple as eye movement are used... you have to see it to understand. It's the kind of thing you don't know games are missing until you see it once, and then it becomes impossible to ignore in all other VNs. Just make sure you look up from the text box after every sentence, otherwise you'll miss all the subtle changes.
And then there's the scene pacing, the editing, the music and the timing. Man, the timing! So many cool moments created just with some smart edits and a good musical cue. (Small, vague story spoiler ahead) The Ocean's Eleven style heist sequence - complete with awesome intro cards for each character and the banging soundtrack - is a perfect example. I was grinning throughout the entire chapter. And there wasn't even any porn in it! Just good cinema. Even if the girls or the harem genre aren't your cup of tea, it's worth playing ETERNUM just for the quality it brings to the table.
Still, this is an adult title, so let's go over that part briefly. I will say that the sex scenes are well done, in terms of both animations and writing. The execution is on par with everything else I talked about above, and that says a lot. My biggest gripe is that the MC and 6 out of 7 LIs are 18-20 years old, which makes it hard for me to project myself onto the protagonist. The writing is good, it's realistic, I can definitely see how a bunch of kids their age would act that way. Hell, if I was 10 years younger, I'd love this for sure. But now? Eh. Why do so many adult VNs have main characters in their last year of high school?
The harem aspect is the second part I dislike, because the MC just... doesn't address it. At all. I get it, this is the genre and this is the fantasy, I understand. I don't mind it one bit. What I do mind is how we're just casually getting involved with 7 different girls without so much as a thought. Not a single self-reflection on what will happen when they inevitably find out. I obviously understand that all LIs will be okay with the harem, that's the point, but the protagonist could at least have some inner monologue about it. Or even better, talk to the girls sooner. It's super strange and creepy to me that the guy basically knows that 7 girls are madly in love with him and he just... leads them on? He clearly loves them back, so why not get ahead of it?
That aside, I will end by saying that ETERNUM is most certainly the best adult game I've ever played. In fact, it's among the best stories I've ever come across. Have you ever read a book, or played a game, or watched a TV show to completion and then just sat there, not knowing what to do next? Finding yourself wondering "what now?" Wanting more but knowing there isn't any, so you just feel.. empty. Lost. And then it occupies your thoughts for days on end? That's how I felt after playing 0.5 of ETERNUM. Except I know there's more coming, and I can't wait. Play the game. You won't regret it.