I am a big fan of signing! I started when my DD was 7 months. She is now 16 months and her langugae is quite advanced for her age. She has over 80 signs now, and can say over 25 different words (although some of the pronunciations are a bit dodgy) plus even more sounds for words! According to the bumpf I got from the health visitor the average number of words for her age is about 6 - 10!
Just recently, sure start has recommended its use, and now every nursery in my home town is being encouraged to use it. So my DD is now doing it at nursery too. The research on it is amazing, and shows that it does help children to communicate earlier. Some studies even suggest that it may help to higher a child's IQ!
But that aside, I have found that it has been amazing for our relationship... in comparison to other babies I know, she has far less paddies, because she just tells me what she wants! Right down to, "I want a book", so I get her a book - and if its not the right one, she'll tell me the one she wants! (by signing frog for example, so I know she wants the book with the frog in it!). She tells me when she's tired, hungry, thirsty, has a pain, wants me to play with her, wants me to read a book to her... the list goes on!
presonally, I cannot see any reason for not signing! It really is as amazing as it sounds!!