It has given, and even still gives rise to two contradictory opinions.

Is it Rabelais' or not?
First of all, if he had left it complete, would sixteen years have gone by before it was printed?
Then, does it bear evident marks of his workmanship?
Is the hand of the master visible throughout?
Antoine Du Verdier in the 1605 edition of his Prosopographie writes: '(Rabelais') misfortune has been that everybody has wished to "pantagruelize!" and several books have appeared under his name, and have been added to his works, which are not by him, as, for instance, l'Ile Sonnante, written by a certain scholar of Valence and others.' The scholar of Valence might be Guillaume des Autels, to whom with more certainty can be ascribed the authorship of a dull imitation of Rabelais, the History of Fanfreluche and Gaudichon, published in 1578, which, to say the least of it, is very much inferior to the fifth book.
Louis Guyon, in his Diverses Lecons, is still more positive: 'As to the last book which has been included in his works, entitled l'Ile Sonnante, the object of which seems to be to find fault with and laugh at the members and the authorities of the Catholic Church, I protest that he did not compose it, for it was written long after his death.

I was at Paris when it was written, and I know quite well who was its author; he was not a doctor.' That is very emphatic, and it is impossible to ignore it.
Yet everyone must recognize that there is a great deal of Rabelais in the fifth book.

He must have planned it and begun it.

Remembering that in 1548 he had published, not as an experiment, but rather as a bait and as an announcement, the first eleven chapters of the fourth book, we may conclude that the first sixteen chapters of the fifth book published by themselves nine years after his death, in 1562, represent the remainder of his definitely finished work.