Cambridge/Lighterlife
A lot of people will lose a good chunk in their first week, mainly as the sole source program (stage one) is ketogenic so you use your glycogen stores up (this is ready energy stored in your liver/muscles) and your body switches over to burning fat as its primary source of energy. Glycogen is suspended in 4 times its weight in water thus a pleasing weight loss in the first week. You do of course lose fat too. You can then expect 3-4 pounds a week on sole source after that. It can mean average weight losses of at least a stone a month, which is why it is appealing to people with a lot to lose - it can be rather daunting if you want to shift 6 or 8 stone for instance and once you decide to make that lifestyle choice you want to reap the rewards as quickly as possible.
If you do a diet like this properly, your consultant will want to guide you through the entire program, through the levels. Cambridge you start on sole source but then you go to stage 2 (add a meal - green and white, so chicken or fish and some green veg) then you move over to a calorie controlled breakfast, green and white and your 2 cambridge products, and so on so forth until you are eating entirely "real" food.
If you don't do this thing where you go through the stages it can be easy for the weight to creep back on but there is absolutely no reason why it should if you do the stabilisation process and establish what your lifetime eating pattern should be to maintain your weight loss. Many do not bother with this and then blame the diet when they whack a chunk back on.
That said, I know people who have taken a year or more to lose 6 stone on Slimming World and other healthy eating diet programmes only to put it all back on in half the time - once again, not the fault of the diet at all but the attitude that the diet is somehow over combined with burying one's head in the sand - something that can be a factor in how you got to be overweight in the first place - understandable.
People who do well on Cambridge often find it is a combination of the swift weight loss which really boosts your motivation to continue, the state of dietary ketosis, which really kills your appetite which many struggle with on more traditional healthy eating diets, so they are able to coast along without any major discomfort once the first week or 2 have been completed. Another factor of success for some is that there is little planning involved - no weighing, measuring, shopping or cooking, just your one stop shop that is your consultant's place of business, where you get weighed, buy your products and that's that. Many people come undone on traditional diets when they are feeling hunger and they pile their plates up just a little bit higher until they are sabotaging their own success. The temptation isnt there so much with Cambridge as you are likely to be less hungry once in the ketogenic state and it comes in a single sachet which makes it sort of easy to just prepare it, consume it, and move on with your day.
Cost is a factor to consider - a Cambridge diet product costs £2.10 for a shake, soup, porridge or risotto/rice pudding and £2.40 for a spag bol sachet or a bar.
If you got fat downing a £6 bottle of wine with your dinner most nights and live for your takeaway, or Starbucks coffee each morning etc you can actually find it "pays for itself" in the money you save from not continuing your bad habits. If you got fat by eating too much budget food or hoovering the kids leftovers or attacking the biscuit tin in the office there may not be that financial benefit and therefore the diet can seem quite expensive. Another factor to consider!
Hope that helps a bit with your decision.