Fate/Kaleid Liner Prisma Illya: Oath Made Under the Snow was a great watch and obligatory for any fan of the Fate/Prisma series.
The content of the film itself is quite specific however which makes it quite difficult to recommend to anyone who hasn't seen all 4 seasons of the Prisma anime. With that said, If you are a fan of the Fate/Stay Night/Zero series you may very well find this film quite enjoyable even without prior knowledge of the Prisma series. While the content of the film is a flashback arc of the third main series of Prisma, it effectively acts as a sort
of spinoff Shirou storyline, taking place in an alternate dimension which pulls from the Fate/Stay Night series, especially Heavens Feel. This may sound very complicated, but again, this is a very specific story and is definitely only able to be penetrated by those who have seen Prisma and those with a working knowledge of the Fate series.
The main reason I am specifying this is possibly enjoyed by those with a working knowledge of the Fate series but not necessarily Prisma is that many are turned off by the loli fan-service that plays a large role in the Prisma series. The animation, art and story line are all very strong in this film and there is just about zero fan service. Therefore, it can definitely be enjoyed by a larger audience.
If you are a follower of the Prisma series, you may be disappointed by the lack of fan-service but if you've read the manga, you'll know this is a very serious arc where Shirou takes up the role of protagonist. Along with that, Illya makes only the briefest of appearances and Kuro, not at all. For a series with such a heavy reliance on fan service and the girls , this could be seen a downside but the film really plays into its respectable and serious tone, I think to its benefit. I feel like They could have done a flash forward to the bathing scene that takes place immediately after this arc for a bit of fan-service but they instead take it full circle in a much more well done and respectable way. As a big fan of the fan-service this series offers, the restraint they show in this film is actually quite commendable.
The film itself very closely follows the source material with great animation and art all around. While I'm not quite sure if the film ever hits the heights of Illya vs Saber Alter from the first season, it does consistently pull off sequences on par with the other highlights of the series. I may have expected a bit too much since it is a film and they generally have much higher budgets, but Prisma fans are generally spoiled on the art and animation front , besides the most recent tv series which this definitely rises above and beyond by a mile.
One interesting addition is an extended montage of Shirou collecting the class cards after his fight with Shinji. My mouth was wide open this whole time due to the sheer unexpectedness of it, the great animation and many fun Easter eggs and references to other Fate series in the identities of the installers. It also came immediately after a great battle with Shinji and featured tons of great 2D animation, as opposed to the final battle with Angelica which relied on quite a bit of CG. It was still a fantastic battle , but because of this the montage really stood out and I loved it.
All in all, definitely check this film out if you follow the Prisma series (I'm sure you're already frothing at the mouth to view it) and even if you are just a Fate/Stay Night or Zero fan. I didn't focus on the earlier, slice of life moments much here. The first half could be seen as quite slow but they really beautifully build up Shirou and Miyu's relationship which causes the rest of the film to be that much more tragic. I went with my girlfriend who has no knowledge of the Fate series and even she really loved the nicer slice of life moments in the first half and was really endeared to the characters.