People like manga for various different reasons. Some people like them for the intense action, others for the overwhelming drama. I like this one for being a simple light-hearted slice of life.
The story is simple. Like many other 4-koma manga, it lacks a clear plot. However, I think it doesn't need one, as the cute and light-hearted jokes compensate enough for it. The concept of communication through paper airplanes was also very cute, which added helped into giving a nice feeling to me whenever I read it. This manga never tried to build up a story. It just brings forth new light-hearted jokes, which each of them making me chuckle. Don't start reading excepting to laugh hard. The story may be described as somewhat lacking, but the rest of the manga make up for it.
I love Matcha Haruki's art. She has a completely unique style that I think helps with showcasing the every feature of her manga, may it be story or characters. If I could describe her art style in one word, it would be "cute". Despite it being slightly simple, it is enough to bring out each of the characters' emotions close to my heart. Every panel has enough detail put into them, including backgrounds and characters. Overall, the art helps with expressing the story, and adds a nice atmosphere when reading, with a completely unique art style.
I really enjoyed the characters of Kimi to Kamihikooki to. Although some people argue that the characters are bland and inconsistent, it doesn't really matter when they are as cute as they are. They are not that impressive as characters if they were alone on a page, but when talking about their interactions, that is a whole other story. As you might expect, all of their interactions are light-hearted and cute, and give a warm feeling to me when I see them all together. Characters don't need to have complicated personalities in this kind of setting, they just have to match with the story, be somewhat realistic, and manage to create a warm atmosphere when ever I see them.
In case you didn't know already through all my praise of it, I really enjoyed reading this manga. I simply love everything about it, from the art to story to the characters. Although the story was somewhat lacking, I enjoyed every moment I spent reading it, and is something I would definitely re-read.
Overall, Kimi to Kamihikooki to. is a nice, cute, heart-warming, and light-hearted manga that would definitely make you smile.