My daughter is on the autism spectrum (with Aspergers) and does it. It's not technically certified yet as a therapeutic riding barn, but will be soon. She loves it. She wanted us to buy a house in the town its in so she could go there every day.
I think its very successful in particular with people on the autism spectrum since part of it involves grooming and general care for the horses, which fosters so many social skills in a natural setting. Brushing the horse is very soothing for my daughter and she feels very much at home with the horse. The horse actually seems to have a real connection to her, as my daughter is one of only two people that the horse will behave for. It is quite amazing. It's not cheap, that's the only downside.
I am curious how it would work in the context of your therapy. Do you have an idea of what you think it might help you with?