I was dreading doing the optifast as I had heard so many horrible things about it, but to be quite honest I really didn't mind it at all. I loved the coffee shakes and the vanilla ones I added fresh strawberries too with some ice, so it was actually quite nice. I also added an equal sachet into both the coffee and the vanilla shakes for a little bit of sweetness which made it much more normal. I was also advised by the dietician to also add a tablespoon of benefibre to each shake, to avoid constipation and also (and just as importantly) to bulk it up and thicken it and make you feel fuller for longer.
I was also told that you can add, say like peppermint essence to say make a choc mint shake or any other essences for flavour. I am yet to try this so can't attest to what this is like.
I did suffer bad headaches and was told even though I was drinking lots, I still wasn't drinking enough and was dehydrating which was causing the headaches.
Good luck :)