It is definitely addictive. Heavy users get withdrawal symptoms, and more importantly the body downregulates its receptors and production of its own cannabis-like compounds. That said, I still weigh costs of making it illegal versus legal and have a hard time justifying the costs of illegality.
References:
Budney AJ, Hughes JR. The cannabis withdrawal syndrome. Curr Opin Psychiatry. 2006;19(3):233-238. doi:10.1097/01.yco.0000218592.00689.e5.
Gorelick DA, Levin KH, Copersino ML, et al. Diagnostic Criteria for Cannabis Withdrawal Syndrome. Drug Alcohol Depend. 2012;123(1-3):141-147. doi:10.1016/j.drugalcdep.2011.11.007.
Rotter A, Bayerlein K, Hansbauer M, et al. CB1 and CB2 receptor expression and promoter methylation in patients with cannabis dependence. Eur Addict Res. 2013;19(1):13-20. doi:10.1159/000338642.
Morgan CJA, Page E, Schaefer C, et al. Cerebrospinal fluid anandamide levels, cannabis use and psychotic-like symptoms. Br J Psychiatry J Ment Sci. 2013;202(5):381-382. doi:10.1192/bjp.bp.112.121178.
Anthony JC, Warner LA, Kessler RC. Comparative epidemiology of dependence on tobacco, alcohol, controlled substances, and inhalants: Basic findings from the National Comorbidity Survey. Exp Clin Psychopharmacol. 1994;2(3):244-268. doi:10.1037/1064-1297.2.3.244.