Quetiapine is a relatively new antipsychotic in the "atypical" set of antipsychotics. Other atypicals include Zyprexa and Risperdal. These differ from "traditional" antipsychotics in that they are active at serotonin receptors as well as dopamine receptors. These characteristics seem to be favorable in having fewer side effects than traditional antipsychotics and also in having a mood-stabilizing effect.
Seroquel is listed at NAMI, the National Alliance for the Mentally Ill, as being used primarily to treat schizophrenia. However, it appears to be increasingly prescribed for bipolar disorder. This I have learned through my own experience and my sister's.
Recently, I was diagnosed as bipolar II (which is not as severe as bipolar I). The first medication I tried was Seroquel, mostly based on its apparent low risk of weight gain and availability in samples. It knocked me out.
My sister has been recieving intensive therapy at a Children's Hospital in Lansing, Michigan. Recently, her doctor has placed her on Seroquel. My mother says that she has gained 20 pounds, almost all in her hands and face. Interestingly, my mother has heard bipolar disorder batted around about my sister for years, but her therapists had been reluctant to diagnose one so young (6 or 7). She's almost 15 now.
In essence, I believe that Seroquel may be better utilized as a treatment for schizophrenia and maybe as treatment for the manic phase of bipolar disorder.
Sources:
http://www.nami.org/helpline/seroquel.html
Mondimore, Francis, Mark, M.D. Bipolar Disorder: A Guide for Patients and Families. Johns Hopkins:Baltimore, 1999.