There have been a number of scientific studies into the use of ayahuasca, most notably in the fields of pharmacology, neurology, psychology and anthropology. Dr. Rick Strassman was the first to be allowed to administer DMT to subjects in order to study the effects of DMT, from which sprang the popular book "DMT: The Spirit Molecule". Since that time a number of studies have focussed on detailing the short term and long term effects of ayahuasca use, especially in the religious setting (Santo Daime, UDV etc).
I encourage you to read the collection of papers released by the International Center for Ethnobotanical Education, Research and Service (ICEERS), entitled Ayahuasca: Scientific Literary Overview conducted by José Carlos Bouso. It details the studies into clinical trials and short term effects, mid term, and long term effects, as well as the abuse potential.
here's the link: http://iceers.org/Documents_ICEERS_site/Scientific_Papers/ayahuasca/ICEERS2012_Ayahuasca_literature_compilation.pdf
The website is also quite informative, if not a little simplistic (but for its purposes of educating an almost completely ignorant public I think it's OK).
As I said, there's a whole heap of studies out there, but I'll list a few more that I can find lying around right now.
They may not answer your question Ringer, in terms of a direct answer, however to the best of my knowledge, the general academic concensus (drawn from scientific enquiry) at the moment is that it does have enormously powerful effects on human psychology and under particular circumstances there have been dramatic improvements to various health issues, such as depression, anxiety, drug abuse, PTSD and OCD. I'm certain a number of scientists interested in entheogens, altered states of consciousness and ayahuasca itself would say "yes there is evidence", however it's up to reading the academic literature, talking to lots of people who work with and study ayahuasca and of course, experiencing it yourself (woops almost forgot the most important part!).
Ayahuasca in Adolescence: Qualitative Results, Journal of Psychoactive Drugs; Jun 2005; 37, 2, p. 135
Authors: Marlene Dobkin de Rios, Ph.D.; Charles S Grob, M.D.; Enrique Lopez, Psy.D.; Dartiu Xavier da Silviera, M.D., Ph.D.; Luisa. K. Alonso, Ph.D. & Evelyn Doering-Silveira, M.Sc
Clinical investigations of the therapeutic potential of ayahuasca: rationale and regulatory challenges, Pharmacology & Therapeutics 102 (2004) 111 – 129
Author: Dennis McKenna, Ph. D.
Effects of ayahuasca on psychometric measures of anxiety, panic-like and hopelessness in Santo Daime members, Journal of Ethnopharmacology 112 (2007) 507–513
Authors: R.G. Santos a,∗, J. Landeira-Fernandez b, R.J. Strassman c, V. Motta a, A.P.M. Cruz a
Effects of the South American Psychoactive Beverage Ayahuasca on Regional Brain Electrical Activity in Humans - A Functional Neruoimaging Study Using Low Resolution Electromanetic Tomography, Neuropsychobiology; 2004; 50, 1, p. 89.
Authors: Riba, Jordi;Anderer, Peter;Jané, Francesc;Saletu, Bernd;Barbanoj, Manel J
