cathshuck : it's great and really worthwhile.
If you speak to any speech therapist who's up to date with current research then they will back up the findings that baby signing improves baby's communication.
The window of baby's beginning to sign is usually between 10-14 months, so if you've been signing for a while and not seen anything back, then persevere.
Once a baby can clap and point, then they can sign.
What parent doesn't teach their baby to wave? Another sign!
So just expand the vocab slightly. You don't need millions of signs and they don;t have to follow a book or a course (although they're a good start).
Just make up stuff, keep it consistent and ALWAYS say the word or phrase at the same time - never sign in silence, because it's not a language - it's extra gestures and actions.
Both my 2 did their first signs around about 9 months and got about a dozen or so in regular use. But as soon as they could say the words, the signs were dropped.
Ds and dd were both early and competent talkers. And pretty chilled out because they didn't get frustrated as much - I knew when they were "finished" or wanted milk or wanted to go home or were looking for something.