I have yet to meet a person who has obtained any internal benefit from doing this and most don't have a good grasp of postures or forms.
Having said that I am sure there are people in the word who can and have obtained this but they are like a grain of sand in dessert.
My answers have always been:
- A DVD is good to reinforce a lesson between classes but not for learning true taiji
- If you can't commit to a class it is doubtful you can commit to a set time to follow a DVD
- The true essence of taiji (the transmission) is obtained by your teacher correcting you and sharing his insights and you feeling his internal development
- Correcting bad habits in taiji can take an extremely long time once you have them.