The P90X workout is pretty effective if the program is followed strictly. Your diet also plays a huge role in determining how fast you lose your fat. Generally, you should be eating around 500 calories below maintenance per day to record any fat loss at all.
Any work out.... Do squats,push ups, crunches, oblique crunches, sprinting, jumping a rope, eating better, sleeping better, alot of water, swimming, and much much more to lose weight. Worm outs vary for EVERYONE.