It's a great system. I've done it twice now, a year apart. The first time I didn't follow the diet and got decent results. I ate healthier just not as healthy as the diet suggests. A year later (after I had gained all the fat back due to inactivity) I did it again and followed the diet with much better results. Read the booklets that come with it and it explains why it changes the percentages of proteins, fats, and carbs. If you're a naturally skinny person and your family is the same, you will probably see results without the extreme diet. But I'm 32 not 18 so the diet plays a big part for me.
The major problem with the program is that it ends. It's very difficult to continue at that pace unless you have nothing but time and dedication. You'll have to either continue to do the program or find another means of working out after you've completed the segment. I've been doing the lighter versions mentioned in the book because I have not time to get to a gym daily.
As far as supplements. They're good and aid in recovery and I recommend them but you don't necessarily have to spend the cash on the beach body supplements though. Go to your local GNC and signup for their gold card for the discounts. Just look for something nutritionally comparable to the beach body supplements if you're following the diet. Otherwise, just ask the meathead at the counter what he uses.