A lot of the liquid diet shakes are loaded up with plenty of vitamins and protein, but I don't think a person should try to live on them for an extended period of time.
It's healthy in the way that you get all you RDAs in on the meal replacements, as long as you follow the NICE guidelines then it is deemed as perfectly safe
I wouldn't try to live on it either. You will lose a lot of weight though. Not my thing really. I'd only do this as a cleansing and a detox. A few weeks would be pretty tough for me!