The Jeanette Jenkins' DVD Core and Stretch It Out is actually two separate workouts: Core focuses on your abs and back; Stretch It Out is entirely stretching. Both workouts run approximately 30 minutes each.
My general outlook on ab workouts is that they're something to endure rather than enjoy. But Jeanette's Core workout, though challenging, is not one that had me counting the seconds until it was over. Plus, it's very thorough, hitting all areas of the core, including the obliques and the back (too often neglected).
But what I really love about this workout DVD is the Stretch It Out section. Stretching is a component of fitness that I need to attend to more; but most yoga DVDs bore me to tears, and most non-yoga workout DVDs just tack on a short, ineffective stretching sequence at the end. Jeanette Jenkins' Stretch stands out as thorough, invigorating, and not at all boring. It's the perfect add-on after a cardio workout, or any time you'd like a good stretch.
Reviews here and here.