Hard to rate this. The science is good, once past the premise, and a darn sight better than the other alien invasion and body snatcher plots of the seventies. The female main character, though, brings the rating down. I cringe while reading lines like "Oh, I am being weak and womanish." Yes, attitudes were different 60 years ago, but I've read plenty of pulp where the female characters were either differently shaped men or completely ignored - either is preferable to the Heinlein approach.
Note to self - time to re-read Friday. Written 33 years later, and I think I remember a much better female lead.