I had to add my dissenting opinion to the otherwise glowing reviews here. There may be some people who will find it helpful or informative.
What NJVC has done here is technically very impressive. His edits are not simply "seamless", they are artful. Audio crossover fades keep things moving and provide a sense of continuity between scenes which otherwise might be lacking. The song choices are inspired and do SO MUCH to increase the enjoyment of the film. There is a lot to appreciate in this edit.
The areas I feel differently than the other reviews basically fall into two categories.
This film is based on one of my favorite books as a kid, and I love the framework of that adventure. A boy goes off on a scary adventure where he 'out-wilds' the wild things and becomes their king before going back home. There's a sense of a grand journey that builds in scale. I feel a lot of that is lost with the restructuring of the scenes and the cutting back and forth in time.
The original film did a great thing when it framed Max's wildness as A) acting out due to the family breaking down and B) a selfish problem to be outgrown. This allows the film to not only be an adventure, but to also be a tale of heartbreak and redemption, where you see Max unable to take his new family situation and run away, then it becomes a coming-of-age story where he comes to terms with the fallibility of adults and the idea that there "is no such thing as a perfect king". He learns to think of others, stop expecting so much, and value teamwork and building something long-term.
For me, the structure of the new narrative does a great disservice to the themes of the story. The pacing is totally thrown off, with Max just suddenly being on an adventure but without the sense of danger and desperation and angst he originally brought to it (which is reflected in the Wild Things, like all his feelings). Then the constant flashbacks in the middle of the film are so on-the-nose paralleled to their real-world components, it doesn't feel like Max has run away from his problems, it feels like he is playing in his own world, consistently imagining monsters and adventures while at home. The ending is the worst of all. It undoes the mother's forgiveness and understanding, and changes the film from Max growing and learning a lesson to Max just having a string of creative adventures. By moving a scene from earlier in the film to the end, the interacting between Max and Mom implies no lessons were learned by either, and the whole thing was just one event in a series. You could argue that it actually doesn't make any sense, but I'll stop at saying that it just doesn't fit with the narrative throughline of the film.
The original film was imperfect. It definitely needed a bit more of the emotive magic that NJVC's musical edits put in. It also probably needed a bit more fleshing out of the story with some more explicit details to add depth. For me though, this edit goes the opposite direction and makes the film very superficial and just 'a boy's adventure'. It's worth a watch, to be sure, and everyone can then make up their own mind. But for me the original film is a much deeper and more emotionally cathartic journey.