I use sign with one of our daughters who tested as profoundly retarded and she doesn't speak. She hears, but she doesn't speak (except to say Mama), and she also seems to process what I say better if I say it in sign, or speech and sign. She doesn't have a huge vocabulary, though.
I also used it our son who didn't talk until after he was two. It cut down on temper tantrums immensely.