I have been fortunate enough to see this program, albeit in the black and white version that seems to be the only one extant. It is the only musical that Stephen Sondheim has ever written specifically for television, and although the ending is a bit abrupt, overall it is a fascinating hour of television. "Evening Primrose" is based on a short story by John Collier, and it's usually described as a horror story, though it's horror in the way many of the episodes of "The Twilight Zone" could be called horror. In other words, there is no blood or violence to speak of, but an overall feeling of menace.
What the program does have is four wonderful songs by Stephen Sondheim, including two of his most beautiful and haunting, "I Remember" and "Take Me to the World." It also has a terrific leading performance by Anthony Perkins, who never had as many roles of this caliber as his talent warranted. Charmian Carr isn't quite as charasmatic as Perkins, but she sings beautifully and certainly looks like the kind of girl Charles Snell could have fallen in love with.
It's too bad this program is not commercially available, and not at all available in its original color version, but there are good recordings of the show's songs, particularly on a CD shared with Sondheim's "The Frogs." The songs are all worth hunting down, and the program itself is worth seeing if you ever get the opportunity.