The visual presentation, writing, and character development are all above average, but not stellar, particularly at the beginning. All these things do visibly improve over the course of the game, and it is cool to see the progression of Caribdis' skill at posing/animation, environment design, and framing, especially in the final act of the story.
It's worth noting that a) this was my second playthrough of the game, and b) I did play with a walkthrough mod, because I do not care to miss out on scenes with a character because I clicked the wrong dialogue option at some point. I'm primarily a VN player, I mostly just want to click and get a good story with sexy moments, and with the mod that's what I got.
I just played Eternum before starting this game, and I can't say that Once in a Lifetime belongs in the same echelon of quality, but on the grand scale of AVN games, it still earns an "excellent" from me. Where Eternum is a great story with sexy elements, Once in a Lifetime is more like a typical porn game that also has a solid story.