TLDR: The movie does a great job of recapping the anime, but have no fear nothing important was left on the cutting room floor.
Though the movie is a recap of the anime that had aired in 2017, the movie does a great job of bringing the entire experience together. With the inclusion of a brand new intro we get to learn a little more about the characters, I won’t spoil who. The episodes were also edited together in a way where anything important isn’t lost, but the slower scenes are cut away. One that comes to mind is the scene with Habo. Otherwise any other changes are along the lines of music, one of the songs is unique and I don’t remember it being in the original anime, taking the place of Hanezeve Caradhina from episode 1. Speaking of music and sound, it was amazing to hear it in a theater with the surround sound. All in all, it was a pleasant experience, where both people who haven’t seen the show, as well as returning viewers can enjoy.
Edit: The first movie also only spans the first 8 episodes with the rest to come in the upcoming movie wandering twilight.
I went to watch this in theaters with 2 friends - 2 of us had seen the anime and the third had not. The question we asked ourselves when leaving was "who was this made for?"
This movie is a recap of season 1 from the beginning to the end of episode 8 or beginning of episode 9. Condensing about 3 hours of anime down into 2 hours means that a lot needs to be cut out. The original 8 episodes don't have an hour worth of fluff to cut, so a ton of minor scenes that gave important background end up getting cut. This ends up messing with the pacing of the story, and everything felt way to rushed. My friend who watched the anime and I kept reminding each other of our favorite scenes that were skipped, and there were a lot. Our friend who hadn't seen it kept making comments like "I didn't understand _____ until an offhand comment later on". He had a lot of trouble following the story, and needed to make a lot of assumptions just to keep up. We filled him in the best we could after, but he said he needs to read the manga to answer all of his confusion.
Even though I had watched the anime fairly recently, there were a number of moments that caught my eye as being jarring due to the bad pacing. Near the end of the orphanage arc, Riko runs off crying, then shows up at the beginning of the next scene maybe 20 seconds later perfectly okay as if nothing ever happened, because several scenes in between were cut. Habo just bursts out of nowhere because any appearance of him in the chase scenes were cut (as was all but 1 other scene with him in it), making that whole part just about Riko and Reg relaxing, then rushing, then relaxing again, and ended by an unintentional borderline jumpscare. I hope you didn't care about Kiwi, because almost every appearance of his was cut, and any background for when he does speak was cut. I feel the movie needed to be at least 20-30 min longer, as too much was cut.
So going back to my original question, who was this made for? It definitely wasn't made for anyone who hasn't seen the anime before, so I highly discourage anyone from recommending this to get someone new up to speed if/when season 2 rolls around. My friend likened it to watching an anime or movie on Youtube that was cut up and jumbled to avoid copyright infringement. As for my friend and I who had seen the anime, we either would've preferred to have just rewatched the anime or wish they had made an OVA instead (which we had never thought of before, but suddenly really hope they make one). I guess it does do what it's supposed to do - it was just a recap. My friend has much harsher words for this than I do and felt it was somewhat a waste of time. I personally did find some enjoyment in it, just not nearly as much as watching the original. Ultimately, I feel this movie is only for the die-hard fan of season 1 who doesn't have enough time to just rewatch it, which is a pretty limited audience.
I have slightly better hopes for the second movie when it reaches the US, as it should be a bit easier to fit only 5 episodes (one of which was double length) into 2 hours, but it will suffer the same problems. If you can afford only an extra ~90 min to just rewatch season 1 instead of watching both movies, you will likely have less regrets.