When I consider whether a show is worth watching there are three main factors I take into consideration: quality, entertainment factor, and the overall message.
Mary and the Witch's Flower was produced by Studio Ponoc, formed by former members of Studio Ghibli and directed by Academy Award-nominated Hiromasa Yonebayashi. It is based off of The Little Broomstick by the Scottish Mary Stewart, which is why it has a very Nothern atmosphere. Though originally released mid 2017 in Japan, Mary and the Witch's Flower only reached Australia on the 18th of January with dubs and subs both available.
Quality
As to be expected from former Ghibli staff, the production quality is excellent. The character designs are well done and modern, the artwork and animation is very impressive and the soundtrack, by Takatsugu Muramatsu who also did the music for When Marnie Was There and Yuasa's Lu Over the Wall, is appropriate and grand. The soundtrack and visuals contribute greatly to the atmosphere and the impression we have of magic. The mark of a well directed movie is if it's easy to follow what is going on. An example of this concept done poorly is The Last Jedi, which was difficult to follow and was not exciting when exciting things were happening on screen. Mary and the Witch's Flower doesn't have any of those problems; the movie flows well and everything is clear.
The character of Mary is the polar opposite of Anna from When Marnie Was There (the Academy Award-nominated Yonebayashi film). Very energetic, tries to do everything, doesn't give up even when she's being a burden, etc. This works well to tell the story, especially since Mary is thrown into a place that she doesn't understand whatsoever. Willingness to try new things and having no earthly idea what is going on is a smart writing technique to introduce the world and important characters since it doesn't have to be preachy or on-the-nose with exposition. However, there is an exposition scene towards the end which didn't need to happen and opened up holes in the plot in the build up to that exposition. Unfortunately, although Mary works well as the lead, none of the other characters seem to have much to them. There are the generic antagonists who want more power but are technically not evil, the boy who will inevitably become important and befriend Mary, a plot convenience talking fox thing that shows up at suspiciously optimal times and solves too many problems for what little character he has, and Mary's grandmother who has two scenes and does nothing useful. Peter, who is an important plot point but a relatively uninspired character who didn't get enough screentime early on, has too much emotional payoff around him for how little he has to his character. This makes the climax slightly awkward.
The writing is on and off. Most of the plot is decent enough, but magic is poorly explained. This shouldn't normally be a problem, because magic isn't supposed to make sense, but the magic academy has some form of magic-infused science which has clear rules. Poorly explained magic in a world where there are rules and scientific principles is not excusable. Inconsistent uses of magic happen a fair amount in the latter half of the film and can detract from the immersion because of how noticeable it is. The motivations of the antagonists is directly related to the theme of the movie, which was that progress at all costs is a bad thing. The film deals with the antagonists in a goofy and whimsical way. There is quite a lot about the film that is whimsical, including character interactions between Peter and Mary, a joke which takes 45 minutes to finally get the punchline, any time Mary uses a broomstick or when there is an escape sequence. It's never a big issue but it wasn't balanced too well with the substantial parts of the movie, which were lacking.
Entertainment Factor
This movie is primarily aimed at children and young girls. For that audience, Mary and the Witch's Flower will be a blast. The magic, thanks to the visuals and sound, is impressive and wondrous. The opening scene of the movie is exciting and the lighting is well done. You can watch this scene on AnimeLab if you have an account. It starts of the movie well, though it's a big tonal shift going from exciting magic to watching an ordinary 12 year old girl do ordinary things. There's enough going on in the plot that you probably won't get bored.
The problems come with the inconsistencies with magic and the plot holes accompanying them. They can ruin the immersion, take you out of the scene and annoy you. The exposition scene towards the end is another mistake because it involved telling us things we already knew. The exposition is likely necessary for children to understand how scenes connect, but the exposition didn't tell us anything we didn't know or anything we needed to know. As far as exposition goes, it was poor. The most fun parts of the movie are where magic is being used to do all sorts of cool things. Luckily, this is the entire second half of the movie.
Overall Message
The core messages were along the lines of "progress at all costs is not worth it" and "you shouldn't change yourself for the sake of change". The messages were clear and shown visually in the latter half. They are well executed because they aren't preachy about it and don't monologue about what the message is and what they learned. For the intended audience, the messages are appropriate and could apply directly to them.
There are some smaller messages, like that you shouldn't abandon what you care about, not to ab use power and that you should take responsibility for the things you cause, but they aren't emphasised at all and are off cuts of the main meal. Of course, there is that one scene where a cow-person is carving up some meat to serve as food, which is quite morally reprehensible. Mary and the Witch's Flower is simple and you shouldn't expect anything else. It does enough to be engaging and somewhat impressive, but not enough to be fantastic and has too many flaws to become a classic or cult-classic.
Should you really watch Mary and the Witch's Flower?
Yes. Yes you should.