"P90X Yoga X" - I vouch for everything they write here.

Serious Work That’s Seriously Different: P90X Yoga X:

"Many people doing the P90X program think Yoga X will be similar to a day off. They think stretching and breathing, maybe some “ommmm”-ing will be the extent of it. The people who think this are in for a shock when they do the P90X Yoga X DVD. This yoga workout is a seriously challenging and very different workout.

...Yoga X seems friendly and non-threatening for about the first 15 minutes. On about your 5th Vinyasa, however, you realize that the ninety minute DVD might not be as forgiving as you assumed. Because many people aren’t used to the yoga style of P90X Yoga X, or perhaps not used to any type of yoga, they may not enjoy Yoga X the first few times around.

There is no doubt that some of the Yoga poses in P90X Yoga X are extremely difficult. You may only be able to get halfway into position at first. This is the beauty of yoga; you do what you can, sinking deeper into poses and breathing. Within several weeks your body begins to adjust to the poses as you become more flexible and loosened up, allowing you to reach and twist further. Yoga is a progressive exercise that allows you to go at your own pace, continually challenging you with deeper, more intense poses.

...You will finish Yoga X knowing full well that you just challenged your body, even without a typical cardio or weight training workout."