Finally got around to finishing this, and enjoyed it enormously.
There's a lot going for this VN, especially if you like cosmic/supernatural horror -- which I don't, particularly, but didn't mind here. There are elements of school dramedy, mystery, and of course romance. The protagonist grows in interesting and more or less believable ways, and I liked his sense of humor. The girls are varied, though they do fall into fairly standard h-games/anime categories (milf/waifu, sexy teacher, bodere, yandere, tsundere, etc.). The incest and harem paths are fun and relatively believable. The follow-through on secondary plot threads and minor character arcs, reintroducing minor bits from earlier in the story, are a lot of fun. The sex is plentiful and varied.
As a visual novel, it's actually got enough choices/game elements that your decisions make a difference in how the story turns out. (I used the mod, and checked all of the possible endings, which are well thought through. I wish the epilogue montages were a bit more involved, though!)
On the slightly less positive side, the visuals are... okay. It's standard, low-res Honey Select models, which mostly bothered me once I figured out that the story was supposed to be happening in Europe (England, maybe?), at which point the fact that the characters mostly looked vaguely East Asian was a bit distracting. (It didn't help that they actually visited Japan. The sequence was fun -- but the fact that the main characters looked just as Japanese -- clothing and hair color aside -- as the Japanese characters was a bit silly.) The female characters are mostly distinguishable by hair and eye color. Pretty old-school, but this isn't a new game.
Honestly? It's a really good, well constructed VN, with literally hours and hours of fun storytelling. It only suffers in comparison to the developer's next game, Eternum, which has (so far) all of the strengths of Once in a Lifetime -- but Eternum has much better graphics, more varied characters, a more compelling plot (the themes are similar, but the world is sci-fi rather than cosmic horror). If I could give it 4.5 stars I would.