Usually musicals in the 1940's were of a set formula - and if you studied films you know what I'm talking about - a certain running lenghth, very "showy" performances that were great on the surface but never got into the real personalities of the characters etc.
THIS ONE IS DIFFERENT - and light years better and well worth it's nomination for best picture of the year - 1945 (although had no chance of beating the eventual winner - Lost Weekend).
Gene Kelly was probably in the best form of his career - yes I know about "American in Paris" and "Singing in the Rain". This one is different. He really gets into his character of a "sea wolf" thinking (at first) that "picking up any girl while on leave" is nothing more than a lark. And if you had to make up a "story" to get her - so be it - until. Sort of like the Music Man when he gets "his foot caught in the door". The eventual hilarity of the film stems mostly from his and his new pal (Sinatra)'s attempt to make the "story" good in order to "get the girl" that he REALLY and unexpectedly falls in love with. You are going to have to see the movie to see what I mean.
Besides that there are so many other elements of great film in this one, it's a classic buddy story, nostalgia to a time when WWII was almost over (the war ended about a month after the films release), a realization that a guy that always laughed at life can find out that he really is a great human being, great songs and probably a few other elements of classic film making that I can't think of right now.
Why not a 10? Near the end - at nearly 2 1/2 hours starts to feel a bit long. There is a small ballet number that Gene Kelly does that must have been a sensation in 1945 but seems dated and feels like it just adds minutes now. But overall, this ones a definite winner on every level.