The premise is that you do the same (yoga based) 20 minute exercise regime every day for 21 days (following it on a DVD). He has settled on 20 minutes, because 20 minutes has been shown to be the average length of time you can concentrate and because the first 20 minutes of exercise has been shown to be the most effective.
20 minutes a day seemed a reasonable amount of time and as I do regular Bikram yoga, which is a 90 minute session 20 minutes seemed like it would be easy. It isn't!
The DVD comes with a book, which mostly is the sort of pseudo-psychological stuff these exercise plans are keen on, but along with all that comes with pictures of each of the postures and also alternative postures for some of them if you find the full posture too difficult. This proved to be an invaluable resource!
I would definitely say I would recommend it. Its "only" 20 minutes, but even so it pushes you and you find yourself sweating. For about the first ten sessions my muscles would ache afterwards. Though as I found I have gone on my flexibility has increased and I ache less and I find I am getting better at the postures I struggled with at first. I actually got my toes down over my head in the plough pose today (at which point I was so shocked I promptly fell out of the pose again).
My one complaint is that its quite basic, the DVD is the same 20 minutes exercise regime and an interview. It seems it would have been good to have some more breakdown of the poses on the DVD and / or some demonstration of the alternative poses. However I think its been deliberately kept "basic" as its been designed to try and sell yoga to the stereotypical man - the kind who thinks Yoga is for gggirrrlllsss only and that "real" men do weight lifting.
I have been combining the 20:21 with my usual exercise regime, but would recommend it by itself or in combination to anyone looking to increase their strength and flexibility.