I've never heard anyone dismiss it as Cuban propaganda? The CIA itself owned up to having done this. It is part of the Church Committee revelations in 1975 (the Church Committee was a Congressional committee that was charged with looking into CIA misconduct — they found a lot of it! Some of it aided with testimony from then-CIA head William Colby, who unexpectedly gave a lot of incriminating information, along with other CIA honchos). The only issues in contention were whether the CIA did these under its own accord (i.e., were they "running rogue"?) or with Presidential input (i.e., was it really US policy?). Search for "the assassination of Castro" here. There is plenty of rumor and area for interpretation about the CIA's shadier tactics, but I think it is pretty well-established that the CIA had active plots against Fidel Castro's life (and active interest in "mind control" and lots of other things that sound almost too crazy to believe if we didn't have ample documentation and testimony on the subject).