There was a post on the General Diet Plans Thread under "Slimfast Question" a few weeks back with a lot of info and differing opinions. You might want to check there.
As for me, I've begun exercising and have been using Slimfast for about 3 mos. now, usually only for breakfast but sometimes for dinner too in the form of a shake. I'm not a big morning eater so this works for me. I use the soy protein formula which you mix w/water so this cuts down the calories even more so than the regular milk based one. I use a salad shaker cup to mix it up and it mixes real well w/ice...the flavor I use has replaced my morning coffee. The ready-mixed drinks in the can are pretty nasty and they taste like the can they came in. I drink a lot of water and incorporate friuts/veggies as in-between-meal snacks. I do not feel deprived, I eat healthy foods when I feel hungry. I've lost 27 lbs in 3 mos and am still working at it. So yes it does work to some degree, at least for me. Do I consider this success? Well, success will be keeping the weight off once I get to goal. A lot of people will slam the product and say that it's too high in sugar and low on protein, etc. While this may be true to some extent, I have been under the care of my physician who has given me the green-light to use this product and I will continue to do so until I get to the size, weight, health, etc that I want to be. And yes, I could use it once a day for the rest of my life. And that's my 2 cents on this matter.