I'll try and make a balanced argument over this film here. Firstly, the bad points, which are numerous considering the different audiences the film may be seen by. It is rated as a PG film, but despite being animated and containing no explicit content, and therefore percieved as a film fit for children, it actually contains quite a few sexual references. Inherently, it becomes much more apparent as an adult that this is a romance film. While watching it as a child, I did not fully percieve this or simply thought it was strange, but watching it now I feel these references and the wording in some of the songs are actually part of the film's beauty, and how subtly inappropriate it can be. Though it will likely just go over children's heads because of that subtlety, it will probably bore them or confuse them. Additionally, the voice acting is quite fine and low in some places compared to the volume of the music --not unlike many other films, but still annoying --and the voices don't always seem to match the lips of the characters moving, which does give it an overall strange appearance. There's also reason to believe that the visuals and art style, which seem charming at this age, may seem eerie, haunting or genuinely scary to little children. Overall, be aware if you are going to watch it with little kids.
Now, why I gave it five stars. The plot is very straightforward and the characters are incredibly humane and yet fragile and surreal in their own unique ways. The art style and extravagant appearance of it all, as well as the colour palettes, gives every scene a tone and depth all its own. It transmits the feelings on the screen quite strongly that way --I actually feel cold when watching some of the scenes. The music may seem too loose, not defined perfectly as the songs tend to be in most musicals, but it blends into the story and the vibes of each moment perfectly, emphasizing the emotional intensity and turmoil that the characters go through.
I think it is a very small and little known film, which is part of what makes it so wonderful and special to me. Sure, there are better love stories and better art and stranger characters out there in other films and books and artwork, but this little film is perfect in itself. The simplicity of the plot, the look of it, the alternative and colourful sight of a little-known frame of time in history as the setting, and how gentle, mesmerizing, unique, fun, fragile and precious the characters are makes the whole film a surrealist and bittersweet realm of its own. It exists not to be great or to compete for better plot or better art, but just to be. I get that feeling very little from any film nowadays, but this one really does seem almost free from the expectations of the film industry. A real pleasure to watch and to understand in all its poetic undertones, but you do nevertheless have to be in that tranquil, appreciative and artistically inspired mood to love it.