A systematic review and a meta-analysis of 2002 from Jacobs et al. has found no benefit in taking it. There has been no harm, but no imporvement in mortality, liver structure etc. So safe your money and avoid using it.
Treatment with milk thistle appears to be safe and well tolerated. We found no reduction in mortality, in improvements in histology at liver biopsy, or in biochemical markers of liver function among patients with chronic liver disease. Data are too limited to exclude a substantial benefit or harm of milk thistle on mortality, and also to support recommending this herbal compound for the treatment of liver disease.