I have the sing and sign dvd rickman, and I only used it a couple of times when ds was about 11 months. He just wasn't that interested and I found it fairly dull too. It was presented by a woman though and he really prefers men so perhaps that had something to do with it.
In the last month or so he has been watching Something Special on Cbeebies and he absolutely loves it, I enjoy watching it too. I'm not terribly persistant about using the signs though as he can say quite a few words now, but even just as something to do together for 20 mins it's fairly worthwhile.