I saw Kingsglaive in theaters, and now I got it on Blu-Ray cause I got the Ultimate Collector's Edition of the game. While it ISN'T necessary to watch, a lot of the cut footage from Versus involving Insomnia is in the movie, like Regis and Aldercapt facing off in the treaty room. The CGI footage of Insomnia falling before Noctis learns of it at the inn is directly from the movie.
I won't spoil the ending of the game, but the MC of the movie appears near the very end of the game.
How are you liking the game so far? I haven't had much time to play cause of work but I still know pretty much a lot of the game from the streams I've watched.