This is one of the few manga out there that takes a typical Ecchi scenario where a pretty girl suddenly appears in a guy's life, but then goes out of its way to actually flesh the scenario out and play it to its natural conclusion in a cohesive, logical way. Most manga in this genre introduce the girl with a random theme like, "Oh, I'm the daughter of the deceased demon lord, now I must live here." And then commences to rarely touch upon the circumstances that brought them together. RE:Marina consistently brings up the fact that Marina is from the future, and somewhen out there is a future Rinnosuke waiting for her.
The manga doesn't drag on like most silly ecchi romances do. Make no mistake, the bulk of the material is just pointless Ecchi romance, which I enjoy, but throughout the story the main character never ceases to stop thinking about the circumstances that are causing his situation with Marina to come about. And naturally, those circumstances do all play great significance in how the manga concludes. What I'm trying to say is that this manga has AN ACTUAL DECENT STORY THAT ULTIMATE CONCLUDES IN A LOGICAL FASHION DICTATED BY THE CIRCUMSTANCES THAT CAUSED THEM TO COME ABOUT.
tldr; its good.
Also, my personal theory for why Marina remembers Rinnosuke in the ending:
To understand how the timeline overlaps, you have to remember some key events.
In the final chapter, Rinnosuke doesn't introduce himself to Marina until she's 23 years old. This is an important detail. In the first chapter, Marina tells use they met and started dating when she was only 20, and he falls ill three years later. So this is when Marina goes back in time, AT THE AGE OF 23.
So if the year she turned 23 is when Marina leaves the timeline, because timetravel is theoretically stagnant, then the year she turned 23 is also when she re-enters the timeline when Rinnosuke comes back to get her. So what you have to understand is that the Marina we are seeing at the end there isn't a new Marina with the old Marina's memories. She is actually the old Marina continuing where she left off when she initiated the time travel.