My 15 month old can't really talk yet.
We've done the most basic signing with him and it is SO useful.
(We didn't bother with the weekly classes and endless homework that we subjected pfb dd to ).
I'm in the middle of something and he can tug on the back of my skirt and sign for water, he gets a drink and I cary on without any guessing of what he wants.
He can tell me he's hungry, thirsty, wants milkies, needs a new nappy, is tired, or has seen a cat (that last one is particularly useful ).
And he's starting to ask where things are as well. It's fun.
For anyone with a young baby I really really recommend it.