Dissociatives (e.g., ketamine, nitrous oxide, xenon, phencyclidine, dextromethorphan in cough syrup) are too addictive, too pleasurable, and reliably produce long-lasting delusional states (e.g., as observed in John C. Lilly and Marcia Moore and Tony Hsieh).

It is disturbing to read Marcia Moore's "Journeys Into the Bright World" (http://pdf.textfiles.com/books/journeysbrightworld.pdf) where she recounts that John C. Lilly warned her to stop using ketamine. She died in the woods not long afterward, hypothermia after injecting ketamine, trying to hide her addiction from her family. John C. Lilly himself went clean after decades of delusion, but never fully recovered, mentally.

For a frank summary of the dangers, see D. M. Turner's "The Essential Psychedelic Guide". Before he died in his bathtub (after injecting ketamine), he wrote the following (https://www.ketamine.co.uk/dmturner/index.html):

  "A major concern regarding safe use of Ketamine is its very high potential for psychological addiction. A fairly large percentage of those who try Ketamine will consume it non-stop until their supply is exhausted. I've seen this in friends I've known for many years who are regular psychedelic users and have never before had problems controlling their drug consumption. And I've seen the lives of several people who developed an addiction to Ketamine take downward turns."

  "After about two years of once-per-week Ketamine use I even found that I had developed an addiction. Although it was less severe than what I've described above, it took considerable effort to break the cycle of repeatedly using it, even though I was aware of detrimental effects that it was causing. Since that time I've used Ketamine only occasionally, but find that I must continually exercise a high degree of will power to prevent myself from falling into a pattern of regular use. Amongst those I know who use Ketamine, I've seen very few who can use it in a balanced manner if they have access to it."

On the other hand, there are other classes of psychedelics that are unambiguously good: tryptamines (e.g., psilocybin) and phenethylamines (e.g., mescaline). These drugs produce informative, useful, introspective, challenging mental states and are NOT prone to abuse. These are the valuable psychedelics.