My only experience has been with chiropractic. We're not happy with them, but we HAD to participate with ASH in order to contract with Anthem Blue Cross. They set the reimbursement rates (which have not changed for MANY years).
I will say that their site makes it fairly easy to submit online requests for treatment, and their response time is quick. We don't always get what we request, but that too can be appealed online.
If they are the payor, you get paid a LOT faster using their EFT program. I don't like it, but we get paid about 60 days sooner.