I'm almost halfway through day 3, I don't feel too bad. Managed to shed 6.5lbs in the first two days.
I'm using bowell's diet sheet, which is actually 800 calories (3x200cal shakes plus 200cal vegetables), which I think is a better fit because I need to run 12 miles next week.
My initial fears about the carb content of the shakes were unfounded - I do spike slightly to over 7mmol/l @1 hour post shake, but this drops to 6 mmol/l @ 2 hours, and I spend most of the rest of the time 4.5-6 mmol/l, which is a bit lower than my standard low-carb diet.
Couldn't do without the vegetables though. 200 calories of brocolli and salad is quite a lot, and helps to temporatily fill my belly.