I have done all three routines. P90x, Insanity and STS. They are all very different and they are challenging in different ways. I completed Insanity last week.
Insanity- This is more cardio based. Insanity definitely challenges the cardio vascular system. My cardio endurance went through the roof and my resting heart rate dropped considerably. The weigh training is all body weight based (except the Deluxe DVd- Upper Body Weigh Training). My shoulders gained more definition, my legs slimmed down with all of the pylo moves and my Abs were ripped. All of the workouts indirectly work your abs from so many angles. Insane Abs and Cardio Abs are solid tough routines. Shaun T has a great personality and really challenges you without being overbearing. Insanity is great for overall cardio training and endurance. I truly enjoyed, enjoyed, enjoyed working out with Shaun T. One of the things that i enjoyed most about Insanity is that there is absolutely no equipment needed, nothing to get or pull out- just show up with your body!
P90X- The weight workouts are challenging- especially the upper body workouts. If there is a criticism, it is that I felt that the leg workouts were insufficient. I always supplemented the P90x leg routines with additional leg work. The cardio workouts in P90x routines are good but no where near as challenging as the 2nd month Insanity routines. Stretch X and Yoga x are solid but they are just too lengthy for my taste. I subbed other shorter stretch and Yoga workouts for the P90x routines. Core Synergistics and Ab ripper X are tough and get the job done. P90x demonstrates what fitness gains and muscle definition can be accomplished with a healthy dose of all types of pushups and pullups.
In P90x you have to choose which focus you will have - strength (8-10 reps or endurance 15+ reps). Finding the right weight so that the last three reps are difficult takes some trial and error.
Some people think that Tony is too talkative or don't like his personality but I actually enjoy working out with him and find him funny at times. His sometimes comic relief was a welcomed addition. P90x is a complete program you get weight training routines, ab workouts cardio routines, yoga and stretch workouts.
STS- Is a solid, excellent strength training program. It takes you through all three training cycles - endurance, hypertrophy and strength. With STS the guess work concerning the proper weight selection is removed with the one rep testing that is done. This really helped me to maximize my time and make the best possible selection per exercise. My only very, very, very minor critique of STS is that I grew tired of the same extended stretch and subbed other stretch routines. I had wished that there was more of a stretch at the end of each of the workouts. some of the workouts in Ab circuits are extremely tough but fun. I loved STS and completed two 3 month cycles. The first one was the standard rotation and the second rotation of STS was undulating. Cathe's personality as always is pleasant and she is tough and to the point.
I believe that all three programs are tough and worthwhile additions to any fitness library.
Mixing it up is good and these programs are different enough and challenging enough to provide your body with a healthy shock of confusion so that you make optimal gains.
I hope that this helps.
Regina