Yoga is a great work out - but if you have never done it before, I'd suggest starting out with a class instead of DVD's. If you want to get the most out of yoga and protect yourself from injury it is important to understand the poses proper form and alignment. Doing poses incorrectly can be dangerous or at the very least completely ineffective.
Love my yoga practice. I attend two vinyasa classes and a restorative class each week. My body has gotten so much stronger. I've lost a little weight, but mostly I'm just firming up. And, I'm relatively box shaped, wide rib cage, large busted, never had a defined waist, my hips are not very wide, just sort of straight up and down through my torso. For the first time in my life. I'm starting to"go in" between my ribs and hips.
I think that going to a studio has helped me immensely, learning proper alignment and how to make modifications. My teachers are incredibly supportive and helpful.