S.H.
I know you were looking for others who have used that program, but my experience may help you anyways.
My daughter has been watching the Signing Time! series since before she was a year old. At first, she would watch & seemed to be trying to do the signs along with the videos. But now, even if I buy her a new one, she doesn't do anything along with the show, she just watches. And she may not always sit and watch the whole show through, but she always asks to repeat shows she's watching, so she eventually sees every piece.
Then, when she's not watching the videos, I have caught my daughter doing signs from the new videos that I hadn't seen her do before. So, even though she doesn't actively participate or even watch the whole thing closely, she was learning the signs!
She will be 3 years old next month and I couldn't even tell you how many signs she knows!
So I guess my advice is, let her watch (or not watch) the videos, but don't push too hard. The more she watches, the more she will learn.