Ideal Protein is a weight loss system that is designed to minimise the amount of insulin your body produces. As a result of taking in a low calorie, low carb, low fat and moderate protein diet, your body will go into a state of ketosis. Ketosis is where your body is not using carbs to give you blood sugar; it is burning your body's own fat stores and that fat is broken down into ketones, which the body can function on just as well as glucose (stored version is glycogen) or blood sugar produced from carbohydrates. If it wasn't for the fact that there is a small part of your brain that NEEDS carbs to function, then we could eliminate carbs altogether.
However, the IP diet relies heavily on soy and sucralose. Soy is problematic for many people with estrogen positive cancers, as it's a phytoestrogen. Sucralose is a low glycaemic index artificial sugar, and there are better sugars out there (monkfruit and some of the sugar alcohols (erythritol, xylitol)) that are zero GI response. This means that your blood sugar will rise for a short period after eating a packet, but it will be of short duration and the low caloric intake (IP is normally around 800-850 calories if you're following the diet closely) will keep you in ketosis. Some of the packets contain dairy products (whey and casein are both milk based) and can cause problems with excess mucus and dairy allergies.
A great many people start out on IP, but find it expensive and start looking for alternatives. When you see the group has a thread called IP Alternatives, you'll know that people are sourcing cheaper (and different ingredient) products elsewhere, and still comfortably maintaining weight loss.
Once you understand how the diet works, it's very easy to switch to real food alternatives by adjusting certain macros, and still lose weight!
Here's a link that explains it:-
How Ketosis works