My two nieces never took to dummies, they took to their thumbs and now at 4 and 6 they still suck them when sleeping and my brother and SIL are STILL trying to break that habit.
My DH and I always said we would not give our child a dummy but at 7 weeks after a night of screaming we gave in and she took to it and loved it and that started our YEAR of not sleeping. We only gave her the dummy for naps and bedtime and thru the night she would wake up crying between 5-15 or more times a night...we would go in and give her the dummy and back to sleep she would go. She did not wake up every time the dummy fell out, but she would usually go back to sleep when we gave it back to her...maybe because we were so sleep deprived for so long, that after trying EVERYTHING to get her to sleep thru the night, we finally just took the dummy away and two nights later she was sleeping thru and has been every since. What a difference a full nights sleep makes for a happy mom, dad, wife and husband!!
They can be helpful, but I agree with the others...wean them from it early!!