You can read more about Rat Park in the original scientific report. A good summary is in this comic by Stuart McMillen. The results aren't widely cited in the scientific literature, and the studies were discontinued after a few years because they couldn't attract funding. There have been criticisms of the study’s design and the few attempts that have been made to replicate the results have been mixed.
Nonetheless the research does demonstrate that the standard “exposure model” of addiction is woefully incomplete. It takes far more than the simple experience of a drug – even drugs as powerful as cocaine and heroin – to make you an addict. The alternatives you have to drug use, which will be influenced by your social and physical environment, play important roles as well as the brute pleasure delivered via the chemical assault on your reward circuits.
For a psychologist like me it suggests that even addictions can be thought of using the same theories we use to think about other choices, there isn't a special exception for drug-related choices. Rat Park also suggests that when stories about the effects of drugs on the brain are promoted to the neglect of the discussion of the personal and social contexts of addiction, science is servicing our collective anxieties rather than informing us.