If something is saccharine, it is too romantic, too sweet, in a way that feels silly and not honest. A saccharine film piles on the sad music and the perfect kisses until you stop believing anyone in it is a real person. People use it about cards, speeches, pop songs, and writing. It is not a compliment. It comes from an old word for sugar, so the picture is “sweet” — though actually nobody is talking about sugar in the kitchen when they say it this way. You can still find a rarer, more literal use meaning “like sugar” or “very sweet to taste”, but in ordinary English, especially in reviews, the fake-sweet-feelings sense has almost eaten the other one. It sounds a little like a teacher’s or a critic’s word; a child would more likely say the film was too soppy.