It was really, really good. It started off rather quiet and sleepy, but slowly built into a crescendo with a satisfying end.
The movie bears some resemblance to other movies like The Girl Who Leapt Through Time and Colorful, and even non-animated movie The Lake House, in terms of plot, but I feel that it can still stand on its own. In fact, I thought its delivery and execution was better than those other movies.
The characters are likable. They're believable. And the way the story built it up, you get to like them more and more, and you fall in love with the two of them and their journey. It was really fun to watch.
Much credit should be given to the writer and director. It wasn't rushed, it wasn't anti-climactic, it wasn't boring. It was perfectly paced, and everything felt just right. Most importantly, the audience is engaged.