First of all, the art! If you didn't like this aspect in the anime, you'll be completly amazed when you read the manga version. Yumeka Sumomo did a great job by creating beautiful landscapes and places, just as the anime version did. One aspect that I found very pleasant were the characters; they are all so well drawn and portraited that you'll find them very different from the OVA version - perhaps much better, I must add.
The story itself is fantastic. With just one volume you'll understand the characters feelings, their stories and their interactions with other persons and friends. There aren't just two main characters and their relationship, but two individual persons and more people around them.
Overall, I would say that anyone should read this manga, especially if you have seen the OVA. You will be amazed with the details, the story and even understand some points that weren't well explained in the anime version. It is a quick and wonderfull reading that will make you understand how far love can go; even if it is ten or a million of years away.
