I've never tried this program, but, I'll share this with you.
Back in '85, I had an Lt that was dating this aerobics/fitness trainer and he suggested bringing her in to change things up a bit for us. He wanted to bring her in for a week to lead some "new style" training for us.
We had just finished a MCREST a few weeks before and were in about as good a shape as Marines can be in. We were running 12 miles each day before morning chow with cammis, boots, and 782 gear. We were humping 10 - 15 miles regularly with the occasional 20 miler.
Keep in mind, we were at Camp Margarita, and our 12 mile run started with the ridge. Very, Very Tough. Marines fresh from Infantry Training School would start to drop by the time they reached the top of the ridge which was less than the first mile.
Anyhow, this is a long way of saying, we thought we were in shape. This girlfriend of the LT came in, and we all thought it was kind of cute, she had her music, her tights, and her little steps and all of that.
Her program was similar to the P90x.
What I recall most was that only about 30 minutes into her workout, I was in pain. I looked around, and so was the entire company.
Though we were in shape, we weren't used to working out our entire core so intensely. After she was done with us, we found improvements in our pullups, our o-course and i-course and similar tasks.
I would just do it. If I was 17 or 18 again, and had that program available to me prior to going in, it would be a no brainer.
JB