I got all the books thinking that baby signing was great, but never got past the sign for milk and maybe a few others (tried it around the 6-8 month mark with Bethanie). It really helps that Bethanie speaks loads, or I might have been more interested in it.. I could imagine how much it will help, especially with kids that aren't picking up on the language as quickly (for example, my brother is dyslexic and didn't speak until way over 3, so my mum must have had a terrible time understanding what he wanted etc).
I might try it with the next baby we have!