Flea Collars are pretty effective and do help prevent the dog from getting fleas.
But you should still give the dog or cat a flea bath every 3 to 4 days out of the week or month to kill off any other fleas that can be hanging around other parts of the dogs body.
Flea Collars are not really meant to cure a flea infestation on your dog, cat or other animal but instead the flea collar is meant to prevent further infestation of fleas on your pet.
You still have to give the animal a flea bath, flea dip etc to get rid of and kill off the flea infestation but flea collars do help some and are worth getting for your dog or cat or other animal to prevent fleas.