IIFYM is just some bullshit name some dude gave to the way that people who understand about nutrition have been eating for years.
Any bodybuilder worth his salt knows that to get a million grams of carbs in a day it's way easier to throw some kid's cereal or something in there than to get all your carbs from rice and potatoes.
But, in the same way they always do, idiots took the idea too far and started getting ALL their carbs from sugary crap, completely neglecting MICROnutrients and fibre. So what was once just common sense has become this weird, polarising thing that people will argue about on the internet, while the people who understand nutrition just quietly get on with things.
So yes, IIFYM can give good results, but remember: before the IIFYM label came along, we just called it "eating"...