TL;DR If you're thinking of watching it then go for it, no reason not to. It's basically exactly what you'd expect. Movie was quite boring but really picked up with the final fight which makes sitting through the entire thing worth it. Review will have light spoilers (Marked).

If you're reading this review chances are you have watched the TV show. I knew exactly what to expect going into this movie: Great fight animation, fantastic score, a generic plot that allows to set up the fantastic moments and apparently some emotional sequences. I was not expecting anything that would be incredibly thought-provoking or something that'll keep me awake at night looking through my brain for answers, and it's exactly as expected.

The enjoyment factor in this movie comes from it's animation, music and supposedly from the characters.

Animation: Again, nothing more than what I'd expected. In fact, I'd say I was expecting more, but that's only because I re-watched the Heaven's feel trilogy recently and anything that's not beyond a masterpiece looks inferior to it. The CGI looks good; the shading is great but the movements are quite, well, CGI-like. Nothing that would redact from the experience, unless you're a huge animation nerd like I am, and even I didn't care much about it because I was too busy looking at the good stuff.

THE GOOD STUFF: (THE VIDEO IS A MAJOR SPOILER)

50 seconds of uninterrupted Nozomu Abe sakuga. This has surpassed his work in episode 19 and is now my second favourite cut by him, coming after only the climactic scene from fate heaven's feel 2. It's so exceptionally well detailed and is perfectly complemented by the genius of Yuiichi Terao who leads the compositing and digital team that it made me tear up. It wasn't just this scene however; the entirety of the final battle was pure bliss. Heaven's feel's climactic fights were about who can make the larger explosion, so it was very refreshing to see a martial arts based well choreographed scene from Ufotable's staff. It wasn't just Abe obviously, Masayuki Kunihiro did some amazing cuts, and even freelancers like Hironori Tanaka joined in. The final fight is amazing. I can't say it enough. I'd say it's in the top 20 fights I've seen in all of anime; I've watched a lot of anime fights.

Things that lowered my enjoyment:

1) Characters: There wasn't really enough time to get attached to Rengoku, so the emotional scenes just didn't hit me as hard. Even though I'm an extreme cry baby (I cried when I first watched deku vs muscular) I didn't even tear up here. It was basically: "Show backstory, show epic scene. Now cry". It worked on my sister but that was mostly because she instantly fell in love with his character because he has Daichi's voice who (Haikyuu! spoiler) tragically dies in season 2, so she had that emotional attachment from the get-go.
2) Plot: I've seen it a million times. It's basically a rip off of infinite tsukuyomi from naruto, which in itself is a rip off of the rip off of the rip off of the rip off .....(n times). So yeah, it's gotten a bit stale.

Not the anime's fault:

1) All of the damn spoilers (spoiler...duh): They might as well put Akaza on the cover at this point. It was the only real plot twist in the movie and it didn't come as a surprise to me. what did surprise me though is his banger theme. Very noice.
2) Viewing conditions: Half the experience is in watching this movie in a movie theatre. I couldn't even imagine. I surely would have whipped my nichirin sword out to rub out a few breathing techniques if I viewed it on a big screen. Unfortunately I didn't get to do this. Around 50 thousand votes on change.org, which was enough for tenki no ko, wasn't enough to bring mugen train to India so I could not watch it in theatres.

That's pretty much everything.

IMPORTANT INFO: The order at which I've watched/read fate: route 1 of the VN-> UBW anime-> Heavens feel 1 and 2-> Zero-> Heaven's feel 3. I have not read the VN for the 2nd and 3rd routes or gotten into the other series in nasuverse.

Heaven's feel III picks up right where the second movie left off, and I'm not talking simply about the plot. The visual spectacle that ufotable was able to construct, the epic score by Yuki Kajiura, the beautiful ending song by Aimer, it's all here. I don't think anyone wants to read a review of me going off about how much of a genius key animator Nozomu Abe is or how good the digital effects and compositing staff is at complimenting their animators. So I'll skip to the thing I didn't like as much. The storytelling.

This movie is the climax. The previous movie was the beginning of the end and this is the end. But of course, a 6 hour adaptation of a 20+ hour visual novel will come with pacing issues. Movie 1 was good but felt like there wasn't a good pay off as it was mostly used for build up. Movie 2 blew me away and is still one of the best movies I've ever watched and one of my favourites; everything was handled perfectly even if some content was skipped from the VN. The issue with the 3rd movie is that there is so much narrative dropped onto you that it gets really hard to follow sometimes. Especially the ending (mild spoiler) which ties to the garden of sinners franchise. There is no explanation to how anything happens in the last few minutes of the movie. This movie is less of an animated show, more of a movie that's made for fans of the VN and nasuverse. Is that a bad thing? Depends on the viewer. Personally, I'd say yes. I had no clue what the hell happened at the end because I haven't watched the garden of sinners series. I am a keen observer so I understood the "materialization of the soul" segment, which also had zero explanation whatsoever, so I'm sure there'll be people lost there as well. Ultimately if an anime-only watcher wants the full experience, they'll have to depend on forums for explanations. I did that, and was really satisfied when I got some closure, but I'd rather have gotten that from the movie itself. I'm not talking about a naruto-like 20 minute backstory or explanation, but a brief cut to the previous scene to explain what's currently happening would have made the movie perfect. I hate it when shows mother you and hold your hand to explain every single detail. But Heaven's feel 3 is no mother. It's the dad that went out to buy milk without explaining anything and assumes that you know all the details. This situation would have been even worse had I not watched zero already, because if I hadn't then Kotomine's segment wouldn't have made sense to me either.

The plot itself is great however. This is the route where every loose end and character arc is perfectly tied up. Calling this "Sakura's route" would simply not do it justice. This route does Rin better than her own route. This route has Shirou choosing love over ideals. It's where Kotomine is perfectly fleshed out (assuming you've already watched zero). Rider's character was done beautifully. I just wish the story could be handled better. Also I know the most common response that I'll get to that is- "read the VN, you filthy casual normie".
I judge anime as anime. Not as adaptations. When I rated promised neverland S2 a 3, It wasn't because it's a dog shit adaptation. It's because it's dogshit. Period.

So... after all that, why the high score? Because I fucking love heaven's feel. I love the story. I love Sakura, and with this movie I love all the characters including Rin who I didn't care much for before. I love the art and animation. I love the music. If the movie does not do anything wrong, lost butterfly for example, I will give it a perfect score. But for a movie that unfortunately isn't objectively or subjectively perfect, I gave it the score right below perfection.

That's pretty much it.