As far as meal replacement shakes go, Slimfast is of very poor quality. Yes, it provides a deficit in calories, but doesn't really provide you with anything of value as it's full of sugars and not as high in protein as others. I have used it in the past and I'll go as far as saying that it made me feel terrible hunger pains and cravings. You're better off cutting out food in the old fashion way. If you really want a meal replacement shake, there's one that I have found to be of much better quality but it's hard to find and quite pricey, but it is very high in protein and contains valuable nutrients. It is called spiru-tein from Nature's Parade. It is better in terms of nutrition because it is very low in sugar, high in protein and that's what you want to stop the hunger and cravings. HTH.
Over the past four years, I've had two stone to lose twice as I gain a LOT of weight whilst pregnant. I've found SF has been an excellent way to kick start a diet. If you do it to the letter, you lose weight quickly in the first couple of weeks which is motivates me to keep going and I find it a good way to get into diet mode...other than my one main meal, I can't have any food other than SF stuff so I can't slip up. Its so dull that I can only do it for a couple of weeks but once I then start replacing the shakes with low cal meals, that actually seems quite exciting. I also find it really easy to do with young children as I don't have to think about buying or preparing separate stuff (well, other than buying the shakes & making them which takes about 30 seconds). So, yes, its a good way to start but I think you'd struggle to stick to it for long. I have no idea about the science of it so don't know if Early is right but the shakes certainly taste really sweet and all of their snacks are fake chocolate or crisps. The very first time I did it, I had SF snacks but then realised that it was much better for me to have an apple or a banana or a piece of toast or something else which was 100cals.