I've been signing with DD since she was 6 months old now. She used quite a few signs. Would love there to be a class near us so we could expand it. It really has been great. The only difficulty with doing it on your own is that you have to be disciplined and remember to use each new sign regularly.
I highly recommend it though. The Joseph Garcia books are especially good as you are teaching them British Sign Language which effectively is another language they can use for the rest of their lives, where as some of the other books use made up signs which while effective fade from use quickly.
Emma x