I just discovered the existence of a 5cm/s manga.
So basically this was published in 2011 so its an adaptation of the film. If you ask me, I'll say that the film is way better. I feel that the film should be a standalone and the manga is unnecessary.
You might want to read the manga some time after you're done with the film anyway, just so you feel like you got a fuller story heh. I wouldn't recommend immediately after though, reading the manga afterwards kinda spoils it until you're like done thinking about the film.
A quote from a review here (emphasis mine):
The adaptation follows some of the usual rules in that it uses the script and doesn’t change all that much in the end, but reframes it in terms of the visuals and gives it a different feel as adapted by Yukiko Seike. With Shinkai’s visual works so striking with its color design, a black and white adaptation has its work cut out for it. Seike, for her part, captures the look of the characters well but has a very challenging job and one that she pulls off beautifully in making sure that the focus is more squarely on the characters. I adore Shinkai’s films, but it’s easy to be wrapped up in the overall mood of the moment because of the animation, music and color design of so much of it. That softens what you take in of the characters and you get a very different feel with it. Going to manga form, we get a lot of detailed artwork and scenes here with some great backgrounds, but we get in the characters heads far more and they feel more grounded and real, not quite so dreamlike.
What I feel is that the beauty of the film, the images, the colour, the music, the animation, all these are crucial in making 5cm/s such a great film, and the moment that was translated into manga form, so much of it was lost. For example, the rocket scene in the film was the highlight of part two, but it wasn't any where as near beautifully expressed in the manga, I suppose there was no way it could have been done. The dreamlike quality of the characters are fundamental in building the emotion, in relating to the characters. The characters themselves are struggling, just perhaps what I'll call 'feeling' life, and when it becomes more concrete thoughts like in the manga (as the reviewer mentioned how 'we get in the characters heads far more') it loses that quality.
The strong point of the manga is of course that in a way it better completes the story, it has a more concrete character development and you understand the lives of the individuals in it better. Yet for me, I feel that perhaps the point is that you can't really fully understand, and perhaps understanding in this case cheapens it. Hence I'd say that this manga is really optional, if you're the kind of person who would want more details about their lives and understanding of their feelings better then by all means go ahead.
On a side note, I lol'ed when I saw these comments on 5cm/s related videos on youtube.
"The title 5 Centimeters Per Second comes from the speed at which that piece of junk train moves, being a metaphorical representation of the state in which wintertime public transport is, reminiscent of the poor track maintenance and how even the finest steel tracks slowly rust away."
"If I could, I'd hire both their parents with a stable job so that they wouldn't have to move anymore."
"I had to eat a bowl of nails without any milk to feel like a man again."