Has anyone retained a Hospital/Medical bill auditing service as a cost containment solution? What are the potential pitfalls?

I did this several years ago. As I recall different states have different rules about what a hospital may charge or not charge the entity (audit firm) doing each audit. The bottom line for us was that because of PPO discounts, often per diem discounts, there was rarely enough savings to offset the cost to the plan for the audit. Didn't find this to be a benefit, but thought it was worth a try.