I really like Core Max. It's a bit different from Cathe's other ab workouts because it starts with the lower abs for most segments. I like the use of the stability ball and the medicine ball. Having 3 different 20-minute workouts to choose from, plus three premixes) makes it very flexible.
I'm trying to weed out the ones I'll REALLY use the most, but at the rate I'm going, I should have just ordered the whole darn HC series to begin with!!