I have 20-month old triplets. They love the Signing Time videos. I sit one of them on my lap and help them copy the signs until their attention span wanes, then move on to the next kid. It really seems to be helping build their vocabulary. The videos seem to help me be a good "teacher"/mommy by acting like a built in lesson plan/guide.
Before I started doing this, we were seeing a lot of confusion with signs. For example, signing "please" when they meant "more". Or signing "milk" for nearly everything or anything. I'm seeing much less of this and a good increase in their vocabulary now.
I don't know how old your kids are, but I hope that helps.
Jen