It's mediocre. I like the beginning, but the plot dies in Major Land and there's very little of interest from that (early) point on. It also has second-rate characters, with Mephisto of all people being the most memorable.
Unusually for a TV series tie-in movie, though, it's locked into a specific place in episode continuity (between ep.36 and ep.37). It's not just another standalone movie. Watch it out of sequence and you'll be confused. It comes after Mephisto's been saved, but before Muse has joined the other PreCures.
Anyway, Mephisto's quite fun here. He's still a twonk and he'll have his unfunny moments like running into a lamp post. You'll laugh if you're six years old. You'll snarl if you're older. However his pre-credits sequence is funny, with the wrong bedroom window and a telephone call to the police. He's a loser who wants to be a good father. He just wants to do right as Ako's daddy, but he's a goof and she's the most mature, acid-tongued member of the cast. (Despite being nine years old.) That's a good motivation for the character and it works pretty well in this film. "Being a father is hard. Being the boss of evil was much easier."
The PreCures themselves are pretty bland, but they all get a nice moment. With Muse, it's apologising to the police. With Seiren, it's the bit in the end credits where she and Hummy watch two kittens playing in the street. With Hibiki, it's when she tells Suzu that an argument with your friend isn't the end of the world and you can always make up again. (There speaks experience.) Then, with Kanade, it's her reaction to the villainous male idol group, Trio de Major.
THE OTHER GIRLS: They look dodgy.
KANADE: Hot, but dodgy.
Major Land has a problem. Queen Aphrodite has turned evil and sucked all music from the world, leaving people turned to stone and the musicians mindlessly, silently playing their instruments. This is creepy... but the film doesn't have any ideas on how to develop this. There's a little girl, Suzu, who's cross with her friend Ako. (The film then does nothing interesting with Suzu, who's ultimately just wasted potential. I liked her end credits cameo, though.)
There are formulaic baddies for the PreCures to fight. There's a particularly unsatisfying use of those Miracle Lights that children are supposed to wave in the cinema while watching the film, although fortunately Howling then revives for one last PreCure battle. (They'd defeated him with the Power of Love.)
There's just not much here. It's 71 minutes with minimal content. If asked to summarise this film, you'd be floundering a bit. Hibiki gets a cool scene where she walks out alone to confront Howling, but otherwise it's a sludge of Generic PreCure Stuff. It would have been okay with a strong main cast, but... well, yeah. You could make jokes about Howling coming inside Aphrodite, if you really wanted, but this is the kind of placeholder film where there's little real difference between watching it and skipping it. A week or two later, it'll all be the same inside your head. Hell, I saw it this morning and I'm already vague on what I watched.