TL;DR: An anime that barely connects its mysteries to develop a romance
While Island is trying to be one of the remarkable romance, mystery anime, the outcomes are pretty unsatisfactory, as it barely connects the mysteries around and develops a romance story. I would say Island is an average show, but can't be considered as a masterpiece, as it can have a better story presentation, rather than things I just saw in the anime.
+Kinda dense mystery elements
+It can be considered as a complete story at the end
+The bond between main protagonists is presented
+Usage of soundtrack and music are pretty cool
-Bland characters, especially for side characters
-Strange story pace
-The characters designs are pretty much similar
To begin with, people considered Island as a fan-service anime, but at the end, you'll be shocked or confused to realise that it's not really the case, as at the end, you will need to figure out the mysteries settled by the settings and most likely you may need to compare the story differences and try to figure what Island is trying to present. I think most people who watch Island in seasonal would have this kind of issue, as the story pace is rather strange and may suddenly have plot elements that you wouldn't expect and may even misunderstanding the ideas of Island, like considering it is just a harem anime or a fan-service anime.I could understand why people think like that, as at first, Island is really like a fan-service anime with many unnecessary fan-service or comedy elements. This is a really common error in most anime that they should present the story in a better way, rather than just rush it at the end, anime fans wouldn't be impressed even the ending is a good ending or bad ending, as in general the story is really feeling bland and the characters are easily forgettable.
Speaking of anime characters, although Island tries to focus on the bond between Rinne and Setsuna at first, the efforts are ruined because of the plot twists. This is kinda strange to say that because plot twists usually contribute well to the story, but in fact plot twists are too late to be developed and there's not much character development because of the poor story presentation, like the strange story pace in this anime. Side characters like Karen and Sara are even more bland because in Island, it's either they're acting like a harem character or it just a minor character that barely contribute to the plot and at the end, I don't really know what the side characters are doing to be honest, they're just really bland and lame.
In a nutshell, Island is really trying to bring out a refreshing romance mystery anime, together with the desire that the main characters want to love each other with a few plot twists, but turns out pretty bad due to a poor story presentation. It's really disappointing that with such great potential, Island could be a great visual novel adaptation, but it ends up like important plot elements are packed and rushed at the end and fan-service is placed in the beginning, like a harem anime.
At the end, let's talk about the animation and sound usage. Well, Island does have kinda good animation, like the visuals used in the Island, the sunlight reflected on the clear sea water, but the character designs are looking too similar, same eye and face shape for female characters, they just change the hairstyle and the colours, others are looking the same if you realise that. Great and nice opening and ending songs, soundtracks and background music are used pretty well, they're kinda fit into the atmosphere. Sadly, this is the one only aspect I think Island is doing great.
For those who are considering to watch this, either you watch other anime or you should consider whether you'd enjoy a dense and packed romance and mystery anime or not.
Story: 5.5/10 (D+) (35%)
Animation: 7/10 (B) (10%)
Sound: 7.5/10 (B+) (10%)
Characters: 5/10 (D) (25%)
Enjoyment: 6/10 (C) (20%)
Overall rating: 6/10 (C)