I use the billings method (CM) and have done so since Dec. We have been TTC but as we are trying for a girl we have been not BDing for a few days before O and keep being too early to get PG LOL!
As we are in no rush it isn't a problem and I'm very confident in the billings method as a method of birth control. I guess you have to be prepared for an accident but the pill can fail too. I have very good signs for O so that helps as I definately know after I have O'd and we are right to go for it.
I found the book to be excellent and have spoken to Dr Billings over the phone about my cycle. There are teachers around as well that can help you if you need it.