I have several people on what I call the "P90x advanced" plan... still have them running the cardio, kenpo, xstretch, ab ripper, and plyo x programs but now they are running weights workouts in the gym to achieve muscle growth beyond what P90 can achieve at home. They are all having great results and I think the cardio elements really help to keep them on point with their diet. It's hard to eat well if you live a sedentary lifestyle!
Hit me up if you want some advice on how to bring the weight training element into the gym. You can use a variety of workouts to keep it fresh but there are a few that work better than others for the p90x/insanity programs.