Free! Timeless Medley Kizuna is what it is: a compilation movie of the TV series. Before I watched this movie, I was expecting much more from it. I expected to see new content from the creators since their last work was High Speed! Starting Days that was released in 2015. However, the movie pretty much skipped through the first season of the anime and mainly focused on the second season (Free!: Eternal Summer).
Almost 80% of the movie was reused scenes from the second season of the anime, and a few snippets from the prequel movie. The movie's own soundtrack were also used in the
reused scenes and basically replaced the original music from the TV show. I did, however, liked the opening of the movie. If a person were to watch this movie before watching the 2 seasons or the prequel movie, it would lead to a lot of confusion to the viewer as the movie did not do a great job explaining the events of the prequel movie and the first season, as well as the characters. Aside from that, we only got 3-4 nice bonus scenes, although I expected much more. As a Free! fan, I kinda enjoyed the movie, but I expected more than reused scenes since there was a 1 year gap since their last work.
TL ; DR: Free! Timeless Medley Kizuna is a compilation movie that mainly focuses on the second season of the anime, mostly with reused scenes and new music that replaced the original in the anime.
Okay, first of all, I rated this as though I hadn't seen the rest of Free! already, so that's why it's rated as a 10. The story itself is and always will be outstanding, and so it deserves a 10. However, I'm going to compare it to the rest of the Free! series because I think that's what most people are curious about.
I've seen anime do this before where they'll sum up a season of the anime with a movie. That's exactly what this movie is. You will get a few new scenes in this movies, which is the good part. For example, you're
gonna see a little montage of when Haru and Makoto first met. Are the added scenes necessary to understand the main story? No, but I actually thought they were pretty cool. There's another scene where Asahi walks behind Rei at a tournament while he talks about this "glasses kid" that recommended some mental training books to him in middle school. Rei recognizes his voice and stares at him as he walks away. So, it's those cool little Easter egg things that make this movie worth at least a skim through. You do need to have seen Starting days to get that last Easter egg, though.
Other than that, don't watch this in place of the actual series. You're going to miss a lot of the details that you'd get from the main series. It really does just focus on the main parts of Free!: Eternal Summer. That's another thing: it skips right by season one. You basically get a montage of Rin and Haru, and that's about it. Even if you watch the first season and then this movie, you're still missing a lot of supplementary content that you get in the second season.
Personally, my recommendation is to watch everything else associated with Free! and then skim through this movie to look for the extra scenes. That's what I did, at least. Again, I rated this like I did because the actual main story of Free! is great (you can read my other reviews on Free! to find out why), but this movie is very much a summary and not a replacement for the series. Just keep that in mind if you decide to watch it.
P.S. You do get some new Style Five music in this movie, so that's a bonus.