While I like p90x for those that need to lose weight and gain an overall fitness level, it has it's limits. Muscle confusion only works for so long and you will peak with it and not see any gains after a certain amount of time, that time being different for everyone based on their fitness level when they start it.
I'm a big believer in Wolffs law:
The body conforms and adapts to the intensities and directions it is habitually subjected to.
The other thing I don't like about the p90x is the length of the workout...for maintenance it doesn't make sense. It's not a lifestyle IMO. I know that contradicts my current workout, but my current workout has a set plan and isn't a lifetime.
For an alternative to p90x some may want to look at
www.athleanX.com ; he claims you'll get the same results in 1/2 the time per workout and is sustainable over a longer period of time. I don't use it but I do use a lot of his meals off of youtube.