It's alright. It's not as groundbreaking as I was expecting, but I guess I shouldn't have watched it expecting to see a movie, because honestly in a lot of ways it wasn't one.

5 cm has beautiful animation and art and the music was wonderfully done. The story and the animation for the most part were paced very well, and the atmosphere was excellently done.
That being said, and I don't know if I'd necessarily call it a problem, but 5 Centimeters per Second doesn't really qualify as a movie, and honestly it barely qualifies as a story at all. The point of the movie wasn't really to create a riveting story; if anything, the director was probably going for telling the story with the bare essentials. The movie was all about capturing the little moments in the daily lives of these three characters and how each interact with each other. With all these elements of the art, dialogue, and the shots in general, it asks the viewer to build the story for themselves, or in other words enjoy and interpret the atmosphere that the movie puts forth. That doesn't change the fact that the story lacks a lot of essential components and details, including an actual conclusion.

As a result, it seems unfair to really compare this movie to that of, say, Spirited Away, or really any other movie for that matter, because what 5 Centimeters per Second tried to accomplish and how it went around doing it is so much different compared to that of any other movie I've ever seen.

With that said, while I did enjoy 5 Centimeters per Second and what it tried to achieve, I also don't think it's a complete masterpiece. I appreciate that the director focused on creating the atmosphere and the setting more than anything else, but I really disliked some of the dialogue, or rather how much it dominated the actual content. On one hand, I understand that the descriptive dialogue was supposed to accompany the scenery of the movie, and to a degree, the well-written, descrpitive lines bolstered the story. On the other hand, the excessive amounts of dialogue dominated too much of the screen time, and often times I felt like I was listening to the movie more than I was watching it. This was especially a problem with Story 1 where it was pretty much 20 minutes of straight inner-monologue and not much else.
This might be a bit controversial to say since some might argue that what Shinkai tried to accomplish with 5 Centimeters was probably not possible if this wasn't the medium he chose, but I also felt to a certain extent that he didn't really use even half of the capabilities the movie medium has to offer. Like I said before, one of the best qualities of 5 cm is its ability to capture those moments that emphasize a certain emotion or feeling. But there's only so much straight inner-dialgoue I can take, and 20 straight minutes of it is simply too much with such little context, and I can't help but think that 5 cm would have worked better as a (light) novel or if it could've been a better "movie" had Shinkai taken advantage of the wide variety of benefits the medium has to offer.

Either way, it's a great movie, but it has its flaws. It's a very niche sort of movie in my opinion, so I'm kind of surprised it's as consistently praised and highly popular as it is.