You're welcome to conduct this study and share your findings. I'm sure plenty of people would benefit.
In the meantime, part of this process is doing the legwork and investigating these programs in order to make an informed decision. Read through program websites, read every postbacc-relevant post on SDN you can get your hands on, send emails, make calls. There is no legitimate one-size-fits-all conclusion to whether postbacc programs are worth it or not - some are pure cash cows with low success rates while others accept 75% of their postbacc/SMP students.