There are 2 types of people that follow this type of diet in my mind. Those that make sensible substitutions to keep things fresh. Instead of red meat, something like salmon. Swap rice for sweet potatoes. Whatever. They stay with good quality foods. But on the other hand there's people that will run to McDonalds and grab a double quarter pounder meal and just trim a few calories from their other meals so it all fits come the end of the day. Too many people use IIFYM as an excuse to be a fat kid.

I'll give a similar example to the second type of person, somebody I know personally who was on a weight watchers diet. She was given a certain number of "points" to eat per day (points are calculated off of macros). So for breakfast she'd have an apple, for lunch she'd have a yoghurt cup. She'd be "saving" her points so she could eat whatever the hell she wanted for supper and still have some points left over so she could snack on some chips and cheesies at night. Every single day she'd hit her points goal, but never lost any weight.