I made my best strength and upper body gains, as well as core strength gains, with P90X. An ideal rotation for me would sub a Cathe lower body workout (like Gym Styles) for P90X legs (at least half the time). The cardio in P90X is not strenuous, and is not meant to be, because it is a muscle/strength building program, and too intense cardio can slow muscle growth (and don't get scared! By "muscle growth," I don't mean "bit, bulky 'Chynna' muscles).
I was hoping the Gym Styles would be a nice answer to P90X, but I've found that I can't increase my weights/reps with the Gym Style upper bodies the way I could with P90X. :-( P90X has the perfect, IMO, combo of work and recovery.
I consider it one of my best fitness purchases EVER.