I know one child who started signing at a very young age, and she started speaking early. She, at age three, speaks more clearly than most children twice her age, and with beautiful enunciation. She also has good manners, which is a nice bonus, and begins each question with "May I".
I think it's a brilliant idea, and if I have my own children, I won't do it any other way. As part of my education, I've had to learn a great deal about the larynx and vocal pedagogy, including early development. I would rather give my child tools to communicate than leave them unable to express their needs, save for crying. I can't imagine the frustration those little ones must feel.
And I don't believe that it has to be any harder to transition from signing to speaking than it is to transition from crying to speaking. It takes a firm parent who has the ability to say no if the child refuses to sign. Not really that big a deal.
It's nothing but an asset.