Here are a few reviews of TCDCS with emphasis on different clinical issues: major depression, fibromyalgia, stroke, etc. Sorry these are paywalled, but the full text may be available with a bit of searching.

Caumo, W. (2012). Neurobiological effects of transcranial direct current stimulation: a review, 1–11. doi:10.3389/fpsyt.2012.00110/abstract

Berlim, M. T., Van den Eynde, F., & Daskalakis, Z. J. (2012). Clinical utility of transcranial direct current stimulation (tDCS) for treating major depression: A systematic review and meta-analysis of randomized, double-blind and sham-controlled trials. Journal of Psychiatric Research, 1–7. doi:10.1016/j.jpsychires.2012.09.025

Marlow, N. M., Bonilha, H. S., & Short, E. B. (2012). Efficacy of Transcranial Direct Current Stimulation and Repetitive Transcranial Magnetic Stimulation for Treating Fibromyalgia Syndrome: A Systematic Review. Pain Practice, no–no. doi:10.1111/j.1533-2500.2012.00562.x

Madhavan, S. (2012). Enhancing motor skill learning with transcranial direct current stimulation – a concise review with applications to stroke, 1–9. doi:10.3389/fpsyt.2012.00066/abstract