You may be wondering about how effective ACT is when studied carefully. The Get Out of Your Mind and Into Your Life book has been studied and shown to be helpful in reducing stress and increasing quality of life. Other studies on self-help books or using self-help books in conjunction with therapy are on-going.
It takes quite a lot to become what is called an "empirically supported treatment"; meaning that a number of rigorous research studies indicate that the treatment has been shown to be effective for helping people cope with a particular problem.
In the U.S., ACT was listed in 2011 as an empirically supported treatment for numerous problems by SAMHSA's National Registry of Evidence Based Programs and Practices (SAMHSA is the Substance Abuse and Mental Health Services Administration, a division of the Department of Health and Human Services).
There has been at least one and in many cases several carefully controlled studies on ACT for substance abuse and smoking, anxiety problems (including OCD, generalized anxiety disorder, and social phobia), chronic pain, psychosis, borderline personality disorder, and coping with chronic disease such as diabetes, epilepsy, or cancer.
ACT has also been shown to be helpful for people who struggle with severe mental illness or have more than one disorder or problem (e.g., anxiety and depression); many ACT studies treat people who have the problem of interest as well as other diagnoses. It's also been used with success for reducing worksite stress, and reducing stigma and prejudice.
Researchers have done a considerable amount of research that shows that the individual processes within ACT can be helpful across problems, and researchers care about ensuring that the treatment you get is likely to be helpful for the particular problem(s) for which you seek help.
As of December 2019, there are over 300 ACT Randomized Controlled Trials and over 45 peer reviewed assessments of the ACT evidence base.