Is art therapy really effective? I read the whole Wikipedia article. But I still have doubts. Is art therapy effective? If so, in which ways has it been studied?
Slayton, D'Archer, and Kaplan (2010) review 35 studies conducted from 1999-2007 about the efficacy of Art Therapy. Their review covers a variety of quantitative designs, as well as a few qualitative approaches. 

There are some serious limitations to the research that they reviewed. There is little consensus about appropriate control groups, some studies rely heavily on anecdotal data (taking the form of a case-study more than an empirical investigation), and different studies use very different ways of measuring outcomes. The authors acknowledge these limitations in their discussion.

They do, however, find that "a small body of studies now exists in which art therapy as a treatment modality has been isolated, measured, and shown to be statistically significant in improving a variety of symptoms for a variety of
people with different ages. The range of participants represented in our survey of studies was broad; it included young children, elders, and all ages in between. A range of treatment settings also was represented, including schools,
outpatient clinics, day treatment centers, residential homes and treatment centers, hospitals, correctional facilities, and nonclinical settings." (pg. 115). 

These studies typically focus on a very narrow set of behaviors or symptoms, and thus it's not clear if there is a general pattern indicating in what situations art therapy is effective.