I like it because there's just an endless amount of possiblities with this one and because it contains so many bits and pieces to add onto other workouts. CTX is BY FAR my favorite Cathe series.
You can add the intervals from Step Intervals onto the end of another workout to "go out with a bang".
You can add the step portion of 10-10-10 to the end of All Step for a 30 min cardio workout.
You can combine the cardio portion of Power Circuit with either the lower body/cardio premixes of HSTA or BC or - my favorite - combine PC's cardio with the circuits from Body Max.
You can do 2 upper body parts a day so that you work each group 2xs per week.
You can add the floor work from PLB to LL for a 60 min. lower body workout.
You can add the abs from LL, All Step, Power Circuit, or KB to the end of ANY workout.
You can do All Step's cardio followed by Step Intervals cardio as a substitute for Step Blast (they are both 43 min. and both end with 3 intervals!)
I've often combined the step and hi/lo portions of 10-10-10 with the hi/lo from either SJ&P or MIC.
This series is so versatile - I could go on and on (but I think I already did).
Also, the upper body weight work from this series is nice in that you can go lighter, like Cathe does for an endurance workout - or you can slow the reps and go heavy for a strength-type workout.
(Edited to correct a plethora of spelling errors - hope I caught them all!)