I have done it 4 times. I really did liked it...a lot. I got great results too. I liked Tony too, I know some people found him to be annoying. I did not, I actually liked that he was himself, a bit goofy at times, but gave great pointers too. The weighted/resistance workouts were, I found, cardiovascular too. He moves pretty quick through the moves, so your heart is beating fast. His Plyometric workout is really hard, but it goes by quick and it is effective. Ever since I did my first round, I have gotten PRs in nearly every race that I ran. I think it was due to Plyo. My other favorite workout from the series is Core Synergistics, lots of fun, and I love anything that really focuses on the core.
I think it is different from STS in a way (I have only done STS once). Month 1, you do the same 6 workouts for 3 weeks, followed by a Recovery week. The Recovery week involves, Yoga, stretching, Kempo, and Core Synergistics. Month 2 involves 2 new resistance workouts, but the Leg workout is the same throughout the whole 90 days (which is the one negative I have). No one works legs better than Cathe. Month 3 is stwitching month1 and month 2 for 4 weeks, following up with a recovery week. If you want your push-up and pull-up numbers to go up, P90X will help with that, no doubt.
I am thinking of doing an STS/P90X rotation....alternating each month, but not doing Meso 3.
If you have any more questions, feel free to ask me.