P90X is an ok starting point for people new to fitness, if you can handle the over-the-top fitness narrators on there. But it WILL stop being effective after about 4 months.

The reason it gets such rave reviews is that people who never worked out start using it and seeing results after 2-3 months. This will hold true for nearly any workout program initially, because and untrained body is much more susceptible to losing weight and/or gaining muscle. If you are serious about wanting to be in shape and have fitness be a part of your life in the long run, I suggest going to an actual gym and doing some resistance exercises with REAL weights - you not only burn fat at a much higher weight, but you also build muscle as well.