Do the Rapid shakes say "Very Low Calorie Diet" or VLCD on them? If so - they are ok. If not - Nope. I don't think Rapid is VLCD, but I could be mistaken.
Some people don't mind the shakes, some people do. Go into it with an open mind. I used a brand called "Kickstart" pre op because my health fund paid for them - but I still use them post op when I need to eat AND drink at the same time. I don't mind them at all.
That said, I always blitz them in a blender, always with a good handful of ice, and I tend to add a bit of sugar free syrup too.