J.K.
We highly recommend the Signing Time video's too!! (www.singingtime.com) My daughter had a signing "vocabulary" of around 40 signs by the time she was 16 months. She had Baby Einstein videos too - but would ASK for the signing time video - by signing the beginning of the song "Signing time with Alex and Leah."
Every child is different, and unlike the previous poster - we don't have any problem with her vocalizing the words that she's signing (Milk, More, Please, etc.) she still does both! She's 21 months now, and talks like a 3 year old - using complete sentences - questions etc. Signing is very helpful when she's being shy - or if she's tired or frustrated! (The terrible twos are coming!)
Good Luck - it's a WONDERFUL thing - and you'll be suprised how much you use it too!
J.