I did Get Ripped to the Core yesterday as well. I started a rotation alternating Slim and Lean, GRTTC, and Tony Horton's Sculpt 5-6 with cardio.
I prefaced it with 30 minutes of Cathe's MIC (the warm-up, and a bit of the low impact floor, then 20 minutes of the step).
By the time I was doing the bench presses, I was sweating like a proverbial pig! Those compound exercises are deceiving: they seem easier than they are (as far as how much they are requiring of your body), because they distract you. At least that's my theory!
When I got to the last four reps of the side squats off the step, my legs were threatening to give out on me!
I love this workout! It goes by much faster than Slim and Lean (even though it's about the same length), and seems pretty effective.