For core strength and conditioning, you can't beat Pilates. I take a class 2 - 3 times a week (for the last 6 years) from a good certified Pilates instructor. I never got the benefit from the DVDs because you don't get the instruction that you need at the beginning. Probably now I could do a good DVD, but I still prefer the class.
Please note that I do Pilates IN ADDITION to the strength and cardio DVDs that Cathe offers. It's a great combination! Pilates is NOT sufficient by itself, however.