Many people feel that you don't know what you are getting with a rescue dog.  In fact, if the dog is being fostered by a breed specific rescue organization, the dog or puppy has been fully vetted, and spayed or neutered when age appropriate; the volunteers and staff will know if the dog is good with children, good with other dogs and cats, housebroken, etc.  As with responsible breeders, the rescue always takes their dogs back if there is a problem.