I'm starting from the standpoint of years of yoga practice and having taught Pilates professionally for seven years: needless to say, the boot camp style of "Biggest Loser" isn't exactly what I would go looking for in a yoga tape.
My initial Pilates-teacher bias notwithstanding, this is actually pretty interesting. It takes standard yoga poses and adds repetitions to the movement. What's really smart about it is three things:
1.) The repetitions are movements that don't put a lot of pressure on major joints--you're not moving through the joints in such a way that there is a lot of force, so this isn't likely to cause injury. Specifically, what you're not doing, is rolling through shoulder joints repeatedly, with all your weight on your shoulders, as in most Ashtanga yoga--which does tend to cause injuries.
2.) It's interval training. What that means is that your heart rate goes up and down a lot. Your body, in response, will default to a higher level of calorie burning and oxygen use, so this is a pretty good way to increase metabolism and what we think of as "lung capacity."
3.) It's difficult enough that you don't feel like you're wasting your time. Each workout is only 30 minutes, but it's difficult enough that, if your goal that day is fitness, you're definitely getting your money's worth.
Some negatives: it's filmed on a really bright gym set, which is a little difficult to watch; Jillian doesn't actually do all the poses; sometimes it's hard to see the beginner model when you need to.
Who this is for: experienced yoga folks who want a different kind of more athletic short practice; anyone wanting to add gym-based yoga to their workouts.
Who this is not for: if you have bad wrists, this might be undoable. Your knees also should be good enough to get up and down from the floor without difficulty.
However, this is a fairly safe, more difficult yoga dvd.
Yogi Eric Schiffman has been quoted as saying, "The point with yoga, is to do it a lot." If having another instructor in your dvd rotation, one who isn't airy-fairy, and overly emotional, for days when you just can't go there, then this is worth adding to the mix.