I must state first that I have not read the manga nor watched the anime before I watched the movie so I'd say it's understandable that you weren't fulfilled or satisfied if you have read the manga or watched the anime.

That being said...

This was honestly one of the best movies I have watched.
The story is heartbreaking. The acting is amazing. I just really enjoyed the movie!
Some may have a problem with the plot but honestly it's just that good.
Maybe I'm overhyping it a little bit but to me it is THAT good so it is definitely a must watch!