I agree with what everyone else has said and just want to emphasise that it would be very dangerous to attempt a total meal replacement diet with Ultraslim/Slimfast etc. It says on their packages that they are not to be used in this way.
Incidentally, I found it very hard to lose weight with SlimFast, whereas on Cambridge 1000 plan, which is similar in that you get two supplements a day and a healthy meal, the pounds have just been dropping off.