The P90x Recovery Drink is an awesome product.  It delivers about 40g of simple carbs and 10g of protein to your muscles immediately after the workout, when your body is calling out for nutrients to replenish its depleted glycogen stores.  I honestly buy a tub as I go, I don't go on the monthly auto ship, because I only recommend using it on days that you are in your anaerobic zone.  These would be any of the Day 1, 3, and 5 strength workouts, Plyo X, and Core Synergistics.  I don't use it after Kenpo, Yoga, Stretch, or Cardio X.  You can try to create your own recovery drink by either making non-fat chocolate milk with a syrup that DOESN'T have high-fructose corn syrup. Ironically, chocolate milk has a similar carb/protein ratio, but it will absorb much slower than a recovery drink which has simpler components, so it still isn't ideal.  I read on Facebook that many people make their own drink with a cup of gatorade (which uses the same sugar as p90x drink: dextrose) and half a scoop of whey protein powder.  This is fine, but by the time you buy the components and measure it out, it's almost worth it to just buy the recovery drink!

The macronutrient (carb and protein) content is by the far the biggest reason to drink it in your post-workout window.  By replenishing glycogen, you will be aiding in the muscle rebuilding process, assuring you'll have energy later in the day, won't be as hungry later in the day, and possibly will be less sore the next day.  Secondarily important, the recovery drink has a number of supplements and vitamins in it that will help battle soreness and help build muscle. 

If you want to know more about the Recovery drink and specifically how its components are beneficial to your post-workout nutrition, please read my review here.