Rohypnol is a a benzodiazepine drug prescribed to treat short term anxiety and insomnia. It has the same basic structure as all benzodiazepines, a class of drug that includes Valium, Xanax, Ativan and Halcion. The structural difference between rohypnol and these other drugs is its possession of a fluorine atom and a NO2 group. There are some two dozen benzodiazepines commonly used, and the difference between them is in the replacement of an atom by another atom. These changes cause slight changes in the profile of how they work, but they all have the same effects and side-effects: relaxation, sleepiness, relief from anxiety, muscle relaxation. In other words, they are all sedatives. The difference between rohypnol and these other drugs in effect is that it has a shorter onset of action, and has a relatively shorter half-life, although not markedly so.
Rohypnol is legal for prescription in Europe, but illegal in the United States. In the United States, it is most known by the slang name roofies and is a "date rape drug", due to its quick onset of action, and its ability to produce a black out state with anteretrograde amnesia. However, these are the same effects and side-effects as all benzodiazepines. As far as I know, Rohypnol is no stronger or quicker acting than either Ativan or Halcion. Much of what is reported about drugs like these, especially in their non-clinical use, is anecdotal.
So while Rohypnol might be, and probably is, slightly more likely to produce black out conditions than other benzodiazepines, the entire reputation of Rohypnol as a date rape drug is something that might not be based on its actual medical effects. In other words, there is a difference between Rohypnol, a benzodiazepine that is not that different in structure and effect from some of the most popularly prescribed drugs in the country, and "roofies", an insidious and evil substance that is used for rape and theft. Somehow, with the addition of a fluorine atom and a nitro group, a chemical substance can become "evil". This is a not uncommon view of the world, even amongst medical and scientific professionals who should know better.