I've tried Slim in Six, Slim Series, Power Half Hour and P90X.
Slim in Six and Slim Series are all very similar workouts: no or light weights, high reps.
Power Half hour is a series of 5 1/2 hours workouts that focus on different areas: abs, upper body, thighs, buns, stretch. In the thighs workout, there is a combo of unweighted resistance work and plyo moves. Abs has both lying and standing work. I like these two the best, but don't use them often.
P90X is in an entirely different class from the rest. There are 13 workouts (though #13, ab ripper, is done 3 times a week, after other weight workouts), and a thorough manual to show you how to use the workouts in a rotation. I'm doing the "Classic" rotation (there is also a "lean" rotation which has more cardio and less weight work, and a doubles rotation, where you add 3 cardio workouts to the rotation during the 2nd month, and 4 during the third month.
The classic rotation is as follows:
Weeks 1-3:
Day 1:Chest/ back : done in a push-pull manner, with ballistic stretches done after ever 4 exercises. You'll do lots of push-up variations (6 different kinds) and pullups (or substitutions: I did 1 arm rows and pullovers before I got my bands to use for pulldowns)
Day 2: Plyometrics: lots of jumping forward and backward, side to side. You do 4 different moves, then repeat, then take a 45 second (?) break.
Day 3: Shoulders and arms: my favorite so far! You do 4 cycles of weight work, each cycle consisting of a shoulder, then a biceps, then a triceps exercise, then repeating the 3 exercises. Ballistic stretch break after each cycle. Working the muscle in a cycle like this, and incorporatiing stretches makes you feel like you're doing less work than you are, IMO. With both this and chest and back, I made progress in either reps or weights each time I did the workout.
Day 4: Yoga X : a 90 minute yoga workout. Starts with about 40 minutes of vinyasa (variations of sun salutations, fast movement, strength moves like down dog to plank, then push-up.etc). I'm not often up to a 90 minute yoga workout, so I sometimes cut it short or substitute.
Day 5: legs and back. Lots of different leg moves, none repeated, using body weight or dumbbells forsome moves.
Day 6: Kenpo X: a kickboxing workout. Some folks find it to be "not iintense enough", but I think it is within the context of the rotation.
Day 7: rest or X stretch.
After 3 weeks, you have a recovery week where you focus on core strength and flexibility.
Day 1: Yoga X
Day 2: Core Synergistics : lots of tough core work (like "superman/banana"rolling from "superman" position, holding, then into "banana" position: lying on your back with legs and arms lifted slightly from ground.
Day 3:Kenpo X
Day 4: X stretch
Day 5: Core synergistics
Day 6: yoga X
Day 7: rest or X stretch
Weeks 5, 6, 7 are similar to 1,2,3, but you substitute "chest/shoulders/triceps" for chest/back, and "back and biceps" for shoulders and arms.
I've made more progress strength-wise, appearance-wise, flexibility-wise than with any other workout program I've used, bar none. I think it's due greatly to the fact that the workouts are cycled in such a way that there is enough rest and recovery to allow one to really go hard and heavy on the weight work.
