(Warning: I go pretty deep into spoilers here, so be warned. If you want a spoiler-free tl;dr: Yes, it's excellent, just don't expect a lot of action, but a lot of world building and exposition.)
Harmony leaves you with a certain emptiness after completion. Not the kind of feel good emptiness, where you're just sad the ride is over and just want to continue the adventure, but a kind of sinking emptiness. The kind that makes you stop and just think about the reasoning behind the character's actions. In Harmony's case, you're left considering that, maybe, living in utopia can be a special kind of hell. It follows that really intriguing line of thought throughout the entire movie, providing numerous examples where you are left to decide if you empathize with the trio's dream to overthrow the utopia or see them as entirely selfish for ruining something so perfect.
Where Harmony really does shine is in its world building. From the drop of the hat, you're presented with this society that lives without worry. In this future world you're shown that their lives are driven by a nanotechnology called WatchMe, which monitors all of that individuals basic functions and doesn't allow them to become unhealthy in any way, and AR contact lenses that essentially tell them how to live. It all comes together to create a very interesting and nuanced backdrop for the conflict that arises. I won't go any deeper into the nuances of the world, but you get the picture. It's such an interesting setup, opening up so many possibilities for the story. While I can say that I am slightly disappointed with the ending (more on that later), it's an extremely satisfying journey to that point.
Another department where Harmony excels is in the visuals. One of the greatest weaknesses of utilizing CGI is that seemingly no one can blend traditional animation and CGI together in a seamless manner. In recent years, we've seen progress be made in that area, but even still, without a large budget, there are almost always problems blending the two art-styles. Harmony does an incredible job in that regard also. I had a my own fair share of doubt when I saw all the CGI and hand-drawn backgrounds, but it fit together so seamlessly, that I couldn't help but be in awe of certain shots. Along the same technical topic, the sound design is really good too. Maybe not on the musical side of things, which was mostly bland outside of the ending song. Where it excelled was the SFX and such. A kind of weird example is that they did footsteps really well, it's is kind of a strange thing to notice, but that's just where it stood out to me. There's not a lot more to add to that point, it was just really solid.
Where I felt Harmony came up short was in the abruptness of the ending. We have the climactic meeting, words are shared, and then its basically over. In my opinion, I could have used some more time to show exactly what Harmony's effect really was and why it was such a "bad" thing. We don't even know if it was actually bad or not since all we see are these pillars, which we're led to assume contains everyone's consciousness. It's not a bad ending per se, I just would have liked to see a little more closure, especially considering how much exposition the rest of the plot had.
Another common gripe I see leveled at Harmony is that the main character, Tuan, is boring. It's hard for me to agree with this stance, as we're shown nothing but her struggling with her conscience and the choices placed in front of her for the entire movie. She legitimately was traumatized with what was essentially emotional manipulation from Miach, her lover. She had to throw all their beliefs away when she supposedly died and had to find a place in the society that she was told to hate. Then, in quick succession, she sees her friend kill herself in front of her, receives news that Miach is alive, learns that Miach killed her friend, has her father die protecting her, and she has to kill Miach herself. At that point, I think her emotional detachment throughout the movie is somewhat justified.
All things considered, Harmony is unstoppable where it shines, but questionable when you consider all the exposition dumps and the abruptness of the story concluding. For me, I can overlook such small things and conclude that I loved the movie, but for some, those things may be a draw. If you're considering watching for an enjoyable, carefree experience, I don't think Harmony will do the trick for you. It can get heavy at times and there's not a lot of action to be had, only an abundance of talking and revelations. In the end though, I would recommend it, just to a very specific group of people, since definitely it won't appeal to the masses.