Although no one in my family is deaf, we chose to teach my daughter to sign starting at around 6 months. My mother knows ASL, as do 2 of my aunts, so it was something we could all appreciate and enjoy. By the time my daughter was 1 year old, she could use over 100 signs and recognized several more that she simply didn't have the dexterity to do yet (but has since mastered). She's a little over 2 now and if signing "slowed down her verbal skills" as some would claim, we can't tell. Her teachers at preschool always tell us how amazingly well she communicates, and how clearly her speech is, and how much better she can speak than her classmates. Last night we were practicing conjugating the "be" verbs! She's 28 months old!
We enjoyed watching the Signing Time videos with her, and she still asks to watch them all the time. They are very enjoyable and move at a good pace - not to slow to be boring but not too fast -- you can pick up the signs and remember them all after only a few views.
Both of my wife's sisters have used ASL with their baby girls and so has another friend of mine. It is astounding to see these little girls sign and talk to each other so well, when most kids their age are just throwing blocks at each other! I firmly believe teaching ASL to babies and toddlers spurs their minds into developing abstract thought and reasoning that gives them a great head start. But who am I? Just a dad who has seen what happens when you spend some quality time with your kids trying to teach them something useful... how could that possibly be worth the effort? ;) If anything, it gets parents into the habit of spending quiet time with their kids doing constructive activities, and that's a habit all parents should have.