I am starting week 12 of the lean program today and am currently on week 12 or 13 of Hal Higdon's intermediate marathon plan (to busy/lazy to write my own plan this time around). I've only changed two days in the P90X program, trading out cardio last week and kenpo x this week for the x-stretch. I didn't see the point in another cardio day when I had done an 18 miler on Sunday and was scheduled for either a cross training day or off day for running.
I've gone from 195 pounds to 178.8 pounds since I got on the program, though I haven't been religious about my diet. If I stick with the P90X plan, there aren't enough carbs for me to run well, so I've upped that part of the diet a bit. My muscle definition has gotten much better, but I haven't done any measurements since the first week as I don't care how many more inches my chest/biceps are.
The worst part is listening to Tony Horton. I lost the DVD remote, so I can't shut his mouth. One one DVD, he says "we're burning the gristle today. . . we don't want it!" and then on another he says, "this guy is fit. Pure gristle." Which is it, idiot? He also gives great insight like, "They're sweating like hell. What does that tell you?" Well, Tony, it tells me they are hot. Idiot. Other than him being the most annoying person ever, plugging his recovery drink every time he can, I like the program. My Yoga X day is also the day of my long run, so that will change the next time I plan to do this sort of thing. It's not incredibly difficult from a workload standpoint, but the challenge is just making yourself do it every day for three months straight.
I hope this helps.