We tried it and found it to be incredibly expensive and slower and therefore not as helpful as more traditional interventions. For all their protestations that it's different, it's really ABA but more carefully structured. A good ABA therapist should be able to do what RDI does. A bad ABA therapist will give you what the RDI commercial says all ABA is like.
The most frustrating part of RDI was the online "course" which runs tandem to the therapy and which was NOT free...but which is a multi-step infomercial for the program that took quite some time to complete.
That being said, I don't think it did any harm and we did get some benefit out of it, but the huge turnaround for DS was when we went to a traditional, comprehensive, multi-disciplinary program. It was part of a hospital, covered by our insurance, and included access to a pediatric neurologist, psychiatrist, developmental pediatrician, child psychologist, social workers, an OT and a speech therapist.