Koyori's route finally reveals everything that happened during that incident, and her status and role in the story. That said I was hoping for more information about her past and her route to be a little more epic but Kamipani isn't supposed to be an epic adventure story so I guess that's expected, plus it's the company's debut work.
For me, the routes go from best to worst:
Megumi
Amane
Mizuki
Koyori
Nonoka
Now that doesn't mean that Koyori's route is bad. The above chart is more like the first four clustered near the top, followed by a good amount of distance, and then Nonoka's at the bottom (poor miko-san).
Megumi's arc is the best for me. It's by far the most emotional arc in the game, and her romance with Tomoyuki feels like that destined love that not even the world can stop, and when they finally pull through you celebrate with them. You can really sympathize with her struggle, and I can easily see some scenes getting tears from tear-jerk-prone viewers.. Her arc's only flaw is that the ending is so sudden and has a deus ex machina quality to it, even if it makes total sense. That's the only downer, but everything else about this arc is great.
Amane's and Mizuki's arcs have the most completeness in that they're both great and don't have a rushed ending like Megumi's, plus they pretty much flesh out everything there is to be fleshed with each respective heroine.
Amane's route is the most "pure romance" like of the game, in that the story's only real purpose is the budding romance between Amane and Tomoyuki. It helps that Amane's an awesomely lovable character, and her whole route is really solid, and feels the most complete. The romance between her and Tomoyuki feels "real" and it's one you can easily swallow, accept, and enjoy.
Mizuki's arc also builds a lot of sympathy factor for the heroine, much like Megumi's. The ending's not as solid as Amane's however, but it's pretty close. Incest-lovers can't go wrong with this one.
Koyori's arc. The most plot resolution happens in this one. I rank this one below the other three because the romance isn't as easy to swallow (felt a little bit from out of nowhere), plus it doesn't go as deep into Koyori and her past as I would've liked. Plus her ending's kinda meh as well. That said, it does give resolution to the whole incident and does close the story in a good fashion; just not with as solid a feeling. Plus the very last scene left me with a slight feeling of, "Wait she can do that!?"
Nonoka's arc is by far the most lackluster. The romance feels far more forced than Koyori's, and the whole arc is pretty meh. The only really good parts were Koyori.
As a matter of fact, I liked Koyori better in the other girls' arcs than in her own arc, lol!