It works. My brother uses them, and typically only 4-5 days a week, and they keep him in great shape. He doesn't look like Bruce Lee or anything, but evidently the DVDs have been helping his hockey game and fighting record. IOW they do just what they claim to do: increase athletic performance, give you explosive energy, animal strength. He's been training seriously for almost a decade now, and since getting P90X two years ago, he's told me it's his favorite routine yet.
I've tried a couple of the DVDs with him. I've got more muscle mass than my bro, I'm in great shape, and I lift regularly, but he actually trains. I couldn't keep up with him! Especially the core and abs DVD, oh man, I couldn't even get halfway through! It actually is like having a personal trainer, in that afterwards you're gonna be hurting in places you didn't even know you had muscles. (This was his first workout DVD program btw, and for me too. Maybe they all give you that feeling, idk.)
I say stick with it. It's gonna get you in better shape and make your muscles into lean efficiency machines. But it won't build muscle mass very noticeably outwardly though, so you might want to pair it with some bench presses too. And to get the most out of it, or if your goal is weight loss, you might want to pair it with a running routine, like my brother does. It's a good investment.