If you follow the program to the letter, including the diet guide, you will definitely see results. I'm in week 10/13 right now of my first time through the program. So far, I'm down almost 30lbs and am definitely stronger, and much more flexible. It really depends on the shape you are in now, the drive you have to succeed, and of course genetics. Not everyone is destined to look like Tony Horton. Don't forget, that dude is damn near 54 years old right now, so it obviously will work if you put in the effort.
I haven't followed the diet guide at all, and I'm down 30lbs, I have been eating a set amount of calories a day but, if I want to go out and have a few Fat Tires and eat a nice meal, I don't worry about it. The next time through, I'm going to really lock it down and see what kind of results I can obtain.