Science

Studies have shown positive effects of mindfulness practice on:

stress management
depression
sleep
improved immune function
attention and memory

Additional research on mindfulness and substance abuse and depression:

Mindfulness impairments in individuals seeking treatment for substance use disorders. Dakwar E, Mariani JP, Levin FR. Am J Drug Alcohol Abuse. 2011 37(3):165-9. PMID: 21410417.

Mindfulness-based treatments for co-occurring depression and substance use disorders: what can we learn from the brain? Brewer JA, Bowen S, Smith JT, Marlatt GA, Potenza MN. Addiction. 2010 105(10):1698-706. PMID: 20331548.