A randomised controlled trial of 934 patients receiving immediate or deferred androgen deprivation treatment found a significant reduction in extraskeletal metastases (number needed to treat 25), spinal cord compression (33), and ureteric obstruction (20) in those receiving the immediate treatment.  Overall, survival was significantly improved in patients with non-metastatic disease but not with metastatic disease.
Similar overall survival results were seen in the landmark Veterans Administration Co-operative Research Group studies, which randomised 2052 men to receive either androgen deprivation treatment or a placebo (with this group later being allowed androgen deprivation treatment on clinical progression).  The researchers found no significant difference in five year overall survival (54% v 56% respectively for locally advanced disease and 32% v 20% for metastatic disease).
A meta-analysis of 10 randomised controlled trials of 1908 patients with advanced prostate cancer found no significant difference in overall survival between LHRH [luteinising hormone releasing hormone] and surgical castration (hazard ratio 1.12, 95% confidence interval 0.915 to 1.386). 
More by Thomas and Neal in BMJ. The review also covers NICE guidelines, complications, cost effectiveness, how the newer and older drugs are administered, and more. Here’s a JAMA clinical review of androgen deprivation therapy.
 Immediate versus deferred treatment for advanced prostatic cancer: initial results of the Medical Research Council Trial. The Medical Research Council Prostate Cancer Working Party Investigators Group. Br J Urol 1997;79:235-46.
 Treatment and survival of patients with cancer of the prostate. The Veterans Administration Co-operative Urological Research Group. Surg Gynecol Obstet 1967;124:1011-7.
 Seidenfeld J, Samson DJ, Hasselblad V, Aronson N, Albertsen PC, Bennett CL, et al. Single-therapy androgen suppression in men with advanced prostate cancer: a systematic review and meta-analysis. Ann Intern Med 2000;132:566-77.