On the comment as far as first person talking, I've seen it work rather well. I think, though, I like the approach of Stephen K. Hayes, (not referring to his art in general, but his DvD material), because he doesn't directly address the audience, but addresses the students in the video. It serves to allow them to practice it, and observe them after he's shown them, and lets him get feed back or correct the mistakes that us at home are probably doing likewise, with those on the video.
I own the entire RVD Black Belt home study course, and I personally do enjoy it. It's very informative, and Shihan Van Donk's message is always positive and light in heart.
As far as learning by DvD, I believe it can be an assistance, but not primary. See, not all who use DvD CHOOSE to because they don't want to be with an instructor. I travel 4 hours and 11 minutes a month to make sure I get live training, and our organization encourages it greatly. Mr. Hayes provides as many opportunities possible to train, as does Mr. Van Donk. I am still subscribed to Shihan RVD's catalog, and try and keep up with upcoming events. Even with all the bad politics, I can't help but appreciate the knowledge and maturity of great teachers like him.
-Scott