I'm sooo glad you posted this. I'm really happy with full liquid stage and I have been freaking out about going the next level. I've got a great routine and feeling awesome. And I'm sure it helps that I love my protein shakes.
I had major headaches and I drank a ton of water, for me it was the detox of not having carbs, sugar and caffeine. Good news is that they do go away. For some they say three days, for me it took 6. Things are much easier now, less cravings and lost a fair amount of weight too
I'm a few days from surgery and the liquid diet was rough for me at night the first 6 days. The time went by way too slow, but I've lost 15lbs and feel awesome now. The thing that got me through were sf popsicles and mint tea.