Just saw the movie, and read the manga, they're complementary as was said before : new scenes, different point of views and narration, even Risa who wasn't in the movie at all.
Anyway, the story is great, or rather WOULD have been great if not for a single constant thing : "WILL YOU JUST FOLLOW THROUGH WITH YOUR FEELINGS DAMNIT?!?!"
Like, it's so frustrating to see Takaki constantly stuck in his feelings, he just looked so weak willed. Takaki likes Akari but not to the point of seeing her in the countryside but still thinks about her but wants to move on but the memory impedes his normal life but doesn't want closure through messages but- holy shit dude make up your mind
At least, the bonus chapter (compared to the movie) was a more interesting ending, with the surfer guy representing a free form of love ("I think this kind of nonchalance is necessary at times", "You're the one binding yourself") that was so desperately missing from the movie. Guess the manga author also noticed that when he decided to add that chapter, props to him.
Everything else about the manga, or the movie, is really good though, I'd still recommend for people who like slower paced stories if they're willing to look beyond the characters' inner dilemmas into the core theme : the passage of time.