My exercise partner does slimfast for one meal and it does help her, but she is only trying to loose a small amount and maintain it.
I believe if you keep your calories for what you eat the same as the calories in a slimfast then you would achieve the same goal, weightwise.
I had used it for my breakfast meal, but it did not do anything for me. I was also exercising and watching my other meals. I found I did better by choicing my own breakfast, within reason.