Psilocybin is not addicting. It can be used to treat addictions such as tobacco abuse.

A small pilot study at Johns Hopkins enable 12 of 15 (80%) subjects to remain tobacco free for 6 months as has been discussed in the press.  Prescription medications such as Chantix, the most potent aid for smoking cessation, have a success rate of about 35% at six months.  There are currently 2 other Clinical Trials utilizing psilocybin to treat addiction disorders: one for alcohol dependence and one for cocaine addiction.

• Magic mushrooms help smokers kick habit in small study (Bloomberg)
  All the volunteers returned two weeks later for another round with a higher dose of the drug. They were all offered a third experience, though several declined, Johnson said. The treatment doesn’t involve swapping one drug for another, said Johnson, who pointed out that hallucinogens aren’t addictive.

• Want to Quit Smoking? Eat a Magic Mushroom, New Study Says (Time)
  “Quitting smoking isn’t a simple biological reaction to psilocybin, as with other medications that directly affect nicotine receptors.” Instead, Johnson said, it was the subjective experience the smokers had when taking the psilocybin that changed them— more like a religious conversion than getting a shot of penicillin to cure an infection.

Below is the study from the Journal of Psychopharmacology:

Pilot study of the 5-HT2AR agonist psilocybin in the treatment of tobacco addiction
Abstract: Biomarkers assessing smoking status, and self-report measures of smoking behavior demonstrated that 12 of 15 participants (80%) showed seven-day point prevalence abstinence at 6-month follow-up. The observed smoking cessation rate substantially exceeds rates commonly reported for other behavioral and/or pharmacological therapies (typically < 35%).
Johansen PØ, Krebs TS. 
J Psychopharmacol. 2015 Mar;29(3):270-9.  Epub 2015 Mar 5. 
PMID: 25744618 
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Perhaps by more than plain chance, the article below was released on the same day in a different journal by different researchers.  Yet it helps explain how psilocybin works to break addictive habits.

Brain Regions Affected by Impaired Control Modulate Responses to Alcohol and Smoking Cues.
Conclusions: Our findings suggest that the posterior cingulate cortex (PCC) and right posterior insula, each playing a role in the salience network, are affected significantly by impaired control for alcohol and in turn influence brain responses to not only alcohol but also smoking cues, providing insight to neuronal mechanisms for concurrent use or comorbidity of alcohol and nicotine dependence.
Liu J, Claus ED, Calhoun VD, Hutchison KE.  
J Stud Alcohol Drugs. 2014 Sep;75(5):808-816. 
PMID: 25208199

Psilocybin works to decrease the positive coupling between the medial pre-frontal cortex (mPFC) and the posterior cingulate cortex (PCC,) both part of the the default mode network. (source)

A recent article in The Pharmaceutical Journal, 27 OCT 2014, Psychedelics: entering a new age of addiction therapy,  provides a good overview of the potential of psychedelics for various addictions, the effects of psilocybin on the brain, and the history of current upsurge in psilocybin research.  This is cutting edge research that shows great promise.

A 2014 article in the Journal of Psychopharmacology titled Hallucinogen use predicts reduced recidivism among substance-involved offenders under community corrections supervision concludes that naturistic hallucinogen use predicted a reduced likelihood of supervision failure (e.g. noncompliance with legal requirements including alcohol and other drug use) and suggests that hallucinogens may promote alcohol and other drug abstinence and prosocial behavior in a population with high rates of recidivism.
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A 2015 article in The British Journal of Psychiatry from researchers at Johns Hopkins states "After a 40-year hiatus there is now a revisiting of psychedelic drug therapy throughout psychiatry, with studies examining the drugs psilocybin, ketamine, ibogaine and ayahuasca in the treatment of drug dependence. Limitations to these therapies are both clinical and legal, but the possibility of improving outcomes for patients with substance dependency imposes an obligation to research this area."
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New article in Progress in Neuro-Psychopharmacology and Biological Psychiatry 14 March 2015 titled "Classic hallucinogens in the treatment of addictions"
Highlights
•We review the use of classic hallucinogens in the treatment of addiction.
•Controlled trials support the efficacy of high-dose LSD for alcoholism.
•Pilot studies of psilocybin for nicotine and alcohol dependence suggested benefit.
•Definitive studies of efficacy and therapeutic mechanisms remain to be done.