I've used it for awhile now. I've modified it to run almost year round, by combining it with Insanity, one other work out system for flexibility, the PT and athletic training I developed myself over the years, and the set of weight lifting and strength programs I've gotten from others or developed myself.
Since I've had my back broken and one knee shattered I sometimes have to modify certain exercises, or skip them for others. That's another thing I like about the program, it is easy to modify exercises. It's very versatile and flexible.
I think the program very effective, I like it a lot, and I think the principles upon which it is based are very interesting and sound. My results have been, and are, good.
I didn't use the diet, but one of my own. But I didn't think the diet was bad.
When I have little time for exercise I instead substitute the 10 Minute Trainer by the same guy.
I like that too. It's very convenient.
I think those are very good workout products.
He did a good job. It's the best commercial program I've ever seen or used.
P.S.: By the way I wanted to mention this also, as a little sidenote. My youngest daughter had asthma from an early age. She used to have very violent coughing attacks, with accompanying tachycardia.
Rather than go the traditional medicine route I did a lot of research and eventually ended up following Teddy Roosevelt's program for curing asthma. in addition we changed her diet, got her to play sports (Roosevelt), had her take up distance running (when I was a kid I ran distance), got rid of all processed sugars and sodas, got her outside and into the air and sun a lot more often (Roosevelt), etc, etc.
But my kids also often follow along with me during P90X or 10 Minute Trainer (if we're short on time). Now all of the things we did concerning treating her asthma were important and necessary. They all eventually helped, and now the only time she ever suffers an attack is in the height of winter, and even then her attacks are light and very mild by comparison.
But in my opinion I also credit P90X as being very helpful in suppressing her asthma, as well as strengthening her body and improving up her respiratory function. Dramatically even, I'd say.
I suspect this is because of the emphasis on almost continual movement as well as the heavy aerobic load it demands.
It took her a lot longer at first to build up to the exercises than we did, but once she began to strengthen, her improvement was rapid and for her age she can keep up with us throughout entire routines now.
I think that much improved cardio-respiratory function is a side benefit of the way the program works. More so perhaps than with other programs.