I think it depends on the person and the workout... my grandmother loses weight by doing walk away the pounds, but that doesn't do anything for me. I lost weight with TaeBo, but it doesn't motivate my other family members that have tried it. My brother is more motivated to work out in a gym with his friends (he does it to bulk up not lose weight) but when he can't he uses videos at home.
If you want to lose weight/get fit with DVDs (or tapes) the trick is finding something that works for you enough to stick with it and that challenges you enough to see results.
Some videos that have kicked my butt in the past were TaeBo/Billy's Bootcamp (especially the advanced tapes... oy), Power 90 and Stott Pilates.
I'm not against joining a gym, I just can't right now. Gyms are a bit more expensive than DVDs.
