The movie grabs your attention immediately and doesn't let go until the end. Post WII Germany is one of the main characters here and it's fascinating to see the bombed out buildings and how people had to live after the war. The fact that it was filmed on location in winter only adds to the mystery.
As for Gene Kelly not being an actor but only a showman, watch him in Inherit the Wind, and you may change your mind. His non-dancing roles were not really all that rare, even during his heyday as a dancer.
Pier Angeli, who was Italian, is cast as a German girl. That doesn't quite work for me, as her German isn't good enough for her to have been a native, yet her appealing nature, her huge eyes and her inherent sweetness help her performance as woman whose innocence has been lost.
The corruption and struggle to rebuild in Germany and Austria after the war is something that most people do not really know much about. Other recommended movies that deal with this subject are A Foreign Affair and The Third Man.