I know you are possibly looking for a clear cut answer like "brainy baby dvd's increased my childs IQ with 50 points or something" but since there are no replies yet, I'd take a stab at it
Whether it's Cbeebies, Kowee, or disney junior, they all have their place, barney, teletubbies etc you name it. The problem with TV is that it's not very interactive, children learn by touching and feeling, smelling and putting things in their mouth.
The best way I think to use TV as a great learning tool is: lets say for example you have a Barney DVD that teaches about circles is for younger children (babies) to collect circles before hand, sit with them as you watch the dvd and show them a circle, let them explore it. As your child gets older watch the dvd and go look for circles afterward.
If it's more of a story, explain to them what they are seeing "that little boy is running" (as they get older "Can you run, show me" etc.
If I had to choose a DVD to just park my child in front of it would be a singing/dancing one that would get them moving.