If you liked the series, you'll love this movie! It's another hit for animated movies this year! CloverWorks does a great job animating everything wonderfully in 2-D. I only wish more movies were animated in 2-D AND got theatrical releases. Even if a short release, this movie was worth seeing it on one of the two days it was in theaters in my town!
One thing to note, there are several moments that tug at your heartstrings. Not surprising from an anime movie, but be prepared to cry a few times. I was sure ready to burst into tears a few times, so you may need tissues!
As far as animated movies I've seen this year, this one is also a solid 10/10! I can't think of any real problem I had with it other than that it was pretty confusing at times (but that can be applied to the anime itself). This movie is up there with Toy Story 4, Steven Universe: The movie, and How to Train your Dragon 3 for best animated movie! Sadly we all know it won't get any nomination, (I mean, neither will the SU movie) but it certainly deserves it. :)