I'm a nanny, and the family I work for has this program. It's great, but the 4year old got bored of watching the same thing over and over (repetition is one of the tools used in learning the words) and the 9month old's interest wasn't held for more than 5 minutes. I believe it can be fun and help some children, but it's not for everybody. Maybe if you incorporate it into your routine... everyday after breakfast we watch it... or something like that it would be more effective.
It did help the 4year old learn some new words when she actually wanted to watch it. So I believe it's effective, if you can get your child to sit down and really consistently work with the DVDs.
Alright, there's my 2 cents. We ended up giving up on this, but I think it works for some situations.