If you're a smoker or drinker that cannot go a couple of days without, then you may struggle to actually make proper use of this program and still see results.
Also, I do feel that you'll need a certain amount of self discipline to be able to see this through. I know it's only 3 days (in theory) but what I soon realized is that 3 days sure is a long time on a plan like this!
I guess that's the price you pay for the results promised.
I would also say that younger children may not be able to use this system for various reasons which become obvious as you read the guide, so if you're thinking of getting this for your 11 year old son or daughter, I would probably look elsewhere for an alternative solution.