Heinlein never lacked for ideas. Many were original with him, such as the multi-generational star-ship. Sometimes he took someone else's idea and added his own fillip to it - which is what he does here.
Johann Sebastian Bach Smith is very old, very rich, very stubborn - and caught in the medical straight-jacket of extensive life support. So he conceives of having his brain transplanted - whether the operation is successful or not, he'll at least escape the straight-jacket. So far, an idea done many times before. Now Heinlein adds his own touch, as the 'donor' body turns out to be that of his young, extremely beautiful secretary, Eunice Branca, who was mugged and murdered. When Johann wakes up after the operation, he finds Eunice there in his head, ready to help him adjust to the new world of being very much a female. Is Eunice real, a product of 'body experience'? Or just a figment of Johann's imagination? Heinlein lays clues to this important question throughout the book, but you'll have to read it and make up your own mind.

Given the scenario above, this seems to be a perfect setup for Heinlein's traditional storming of the taboo bastions adhering to sex and gender stereotypes in American society. And there is no shortage of comments, situations, and happenings about just these items. Unfortunately, there is entirely too much of this material, occupying almost all of the middle section of this book, and after the first few sexual situations that Joan (the Johann/Eunice hybrid) faces, becomes extremely repetitious. Joan is not very believable as a woman (female characters were never Heinlein's strong point), nor do her actions really jive with what a 95 year old man would do. The internal conversations between Johann and Eunice are interesting and well done, though here again it becomes somewhat repetitious in the later stages of the book.

When Heinlein leaves Joan's intimate life for a broader look at his envisioned world, it gets much better. The book is set in what he described in other books as "The Crazy Years": illiteracy is common, people need to live in armored fortresses, drive in the equivalent of tanks, court decisions are just as crazy as the one's you read about in today's newspaper, homosexuality is actively encouraged as a way to limit population growth, some areas of cities have been completely abandoned by the police as impossible to enforce. Heinlein's description of ordinary living amongst the youth of the times, his depiction of Eunice's husband Joe as a real artist, his satirical snapshots of the headlines of the day are all excellent, and his headlines are far too close to today's reality to be easily dismissed as 'impossible'.

Heinlein became extremely ill just as this book was going to final edit, and his wife ended up making some of the decisions about the final form of this book. I think that if Heinlein had been well, a large portion of the middle section of this book would have been cut, and some tightening up done on the rest of it. As it is, it is far from his best, even making allowances. But the idea and situation are intriguing (who hasn't fantasized at least once about what it would be like to be the other sex?), in places Heinlein's power to engross and change your world-view are in full flower, his believable world-building skills much in evidence, his messages important and relevant to today's living. Heinlein on a down day was still better than ninety-nine percent of the other material on the racks.