It amazes me that people so often talk about 'success rates' without first defining success...

Consider:
An alcoholic who abstains from alcohol, doesn't get help, falls into a depression.
A drug addict who gets clean but drinks once a week.
An addict who gets clean and sober but slips once without any serious consequences...

It is impossible to have a success rate without any definition of success! Furthermore, there is no reliable way of testing success. Even if they did do follow ups (which they don't) there is no way to know the reliability of the responses (or that there wasn't a difference between those who chose to answer and those who didn't).

Success rates are invented. The only question is if the program works for you... 12 steps do a lot of good for a lot of people but do not suit everybody's needs.