(Spoilers ahead!)
That is to say, it's a light and enjoyable (almost bland, in a pleasant way) experience for the first few summers, before becoming rich and enjoyable (with a slightly bitter aftertaste) at the end.
The story is light-hearted, with occasional bonks on the head to the main character.
The sound is light, and managed to leave a very 'summerish' impression on the listener.
The art is light, the colours also do well to give a feeling of sunshine and brightness.
Over time, things start to feel a little heavier and a little more sophisticated, which goes well with the fact the main character is growing older and the end of the anime is on the horizon. Instead of being a young girl and just playing with Gin carelessly and aimlessly, she starts to become aware of her own feelings and how powerful they are. She begins to lament in the seasons that aren't summer, the seasons without Gin. She misses him, and the moment she realises it, the watcher does too. It impacts both of them, which is a powerful way to create an emotional bond with a character.
On Gin's side, he remains mysterious up until the very end. The wearing of the mask is a wonderful touch, and when he reveals his face (after pretending to be asleep) you get a playful and caring feeling from him, as if he is a worldly embodiment of the complete lack of malice. There are no dark undertones in his mannerisms, which leaves him as a pure and unknown being, one of the things that is easiest to admire and fall in love with. This is definitely enforced by his one and only desire:
Even as he is disappearing, never to experience anything else this world has to offer (after a limited life of only the forest he was abandoned in), the only thing he can do is smile and say;
"I can finally hug you!"
The most romantic, emotional and evocative kind of love, one that is founded solely on emotional and mental connections. The physical aspect at the end is both one of the most important moments for both characters, and at the same time, not very important at all.
The physical part of their relationship is brief and short-lived, but something their lives together have been working towards for a long time.
In a word, it's beautiful!
