Voice of experience trying this: It is a lot harder than it looks, but can be very rewarding in communication. We never got great at signing with our daughter Kat, but we got good enough before she became (ridiculously) verbal that it helped.

My nephew Jude is pretty good at the basics. I know they taught him sign at his school. Although he's quite verbal too Troy says he still uses sign sometimes when he's supposed to be quiet. Sometimes he's just loud as usual. *grin*

I tried this with my son, and while it was difficult for him to follow the hand signals to begin with, he very quickly developed his own IF the ASL one did not work for him. It was very rewarding to be able to communicate with him about his needs before he was able to voice them to me. It has continued to help us since my son has been diagnosed with a "ringing" in his ears that makes it difficult for him to make words with the right sounds (for example, again comes out azzain with a double z sound instead of g). His progress in therapy is amazing and is well ahead of schedule due to the groundwork laid down by signing.