Okay, my personal experience is that baby signing has allowed me to communicate with my child in a way that she could never have done verbally. At a year, she was able to sign really complex things like 'washing machine' (she has an obsession for them), 'chickens', 'light' and 'pain'. Some examples of some of the things she was able to sign to me in the days before she was physically able to say the words:
Driving home one night from ILs. She cried. Tried everything to pacify her. Nothing worked. Stopped the car, pulled over asked her what the problem wass. She signed light. Put the light on, she stopped crying. Turned it off (a bit later), she started crying again and so on. Ended up buying a little push button light as we worked out she didn't like the dark! She was just over a year.
Many a night if she work up crying, we'd go in and ask her what the problem was. When she was teething, she'd sign pain, so we'd get the teethas. Give her some, she'd go back to sleep no problem. Other nights, she'd sign nappy or food. Either way, no guess work involved.
At 12 months, my DD could not just sign book, but could even request the specific book she wanted (book + pigs or book + aeroplane!)
According to my book "Children's communications skills" (written for professionals, not parents!) it says that a child typically uses between 50 and 200 words at the age of 2. My DD was using 140 signs and words at the age of 14 months! That means she was able to communicate (although not verbally) on the same level as a typical child almost a year older than her! Now she is two and she has been involved in research for the baby lab in Oxford. When I last took her in, I had to complete an inventory of all the words she uses (according to their criteria) and she had over 500 words at 22 months! (Which, according to my book is typical for a child aged 30 months!).
I am not saying that she would not have been an early talker anyway, but a couple of effects that I noticed, was that as my DD learnt to sign to me more, the more she wanted to "talk", sign and read books. She quickly learnt to ask what the sign was for new words and I found that as she asked what words were, so I would say them more and she would sign them more - simply the words were repeated more / she heard them more and we engaged in communication more. (This is the reason why the national organisation for SLTS think signing may seem to help children's development, not that it physically helps, but that it encourages the carers to slow down what they say and focus what they say to the child).
Most of her early spoken words were her signs, it seemed she started signing, and then it was the signed words that she spoke first. Once those had been mastered, she then went on to use other words.
The best thing about signing is that I was able to get a glimpse into her life in a way that I could never have done without the signs. As I say, she quickly learnt signs for unusual words like washing machines, and would ask to go and see the washing machine... or the chickens at my ILs house... I quickly knew her favourite books, the ones she always asked for. She'd point out things in the street that she found interesting like ladybirds and aeroplanes, words that a 14 month old child would struggle to actually say. To me, knowing my child at that level and really being able to engage in conversation with my child was really worth it...
I think it is the personal stories like these that convince people rather than the facts as it were... most people I know were sceptical when I started, now my family and friends have seen it, most of them would agree that child geniuses aside, the overwhelming majority of children would not be able to say verbally many of the words that my child was able to sign at 12 - 14 months. Quite simply, it does allow your child to communicate at a much deeper level at that age, which IMO, can only be a good thing!!
Oh, and another thing, avoiding the tantrums caused by not communicating has been a blessing.
I HTH!!