Since everyone else and their second cousin is filling up my notifications bin with reviews for this game, I'll toss in my fifty cents.
From an objective narrative standpoint, I felt like I was reading two well-written stories that had almost nothing to do with each other, yet were being forced to work together. And I cite this critique in the most constructive way possible, so that the dev might consider my opinion moving onto future projects.
At its core, Once in a Lifetime is a story about a young man who is getting it on with his sexually frustrated (not) mother, curious (not) sisters, one of his classmates, and his sexually neglected teacher. All of this could have easily been done without the backdrop of mysterious cults and magic.
Each part of a story should be indispensable. This is why critics hate Jar Jar Binks and Olaf the Snowman - because you can rewrite the whole story without them and accomplish the same thing, if not also do it better. So while I was playing this game, it felt like the fantasy narrative was not as meaningful to your character's relationships with the girls as I would have liked.
Yes, both father and the fiance had probable cause to be absent, giving their significant other's probable cause to feel neglected, but the cause of their absence could have been fulfilled with anything, be it cheating with a side piece, a second job, gambling problems, or any other sort of second life instead of a fantasy about grimoirs and gods. As it presenty is, it feels superficial, almost like creating the connection was a second thought.
Fantasy in general has its own set of problem. I really do enjoy fantasy stories, but I also hate it whenever magic is used as plot armor, or to get the MC to cheat a victory. To Caribdis's credit, there was not a great deal of this, but it did exist.
Once in a Lifetime was also made in such a way where you really only ever have to play the game once in your lifetime. Sorry, I couldn't resist the joke. What I mean is, you can see almost all of the content in a single play-through, if you want to. There are inherent good and bad aspects to this sort of game design that simply are not the dev's fault. Some people like and dislike only having to play a game once, some people like and dislike having to play a game more than once. You simply cannot please everyone. I only bring this up for readers to keep it in mind.
There is a secret ending if you follow no girls' paths, but I don't know what that ending consists of (yet). But this ending from what I can tell is the only ending you'd have to play the game a second time to be able to see.
Ignoring the taboo kinks, the sex options in this game are very vanilla. Oral, vag, and very little anal. It was actually refreshing, in my opinion, and it made sense for this story and the relationships being developed. Lots of other games are filled to the brim with every weird-AF fetish that puts rule 34 to shame, especially the Japanese games. Brownie points for keeping it simple.
In summary: Once in a Lifetime's worst flaw is it wasn't nearly as compelling as I think it could have been. But Caribdis did so well in every other aspect that I cannot bring myself to knock off more than one star. I think everything else has already been said 200 times, at the very least, by all the previous reviewers. This game is definitely above the average, and I am curious to see what else Caribdis makes in the future.