I love HC Extreme. IMAX3 is a tough workout, so pairing it with Low Max makes it more doable (however adding the KM blasts at the end does up the energy level!). And when I want a heart raiser - IMAX3 with the KM blasts - wow!
Plus, all of the IMAX3 intervals are on it, so if you have a programmable DVD, you have IMAX3 as well.