I'm in week 3 of my pre-op diet - the only food I'm allowed to take are 3 to 4 protein shakes per day.
My diet lasts for 3 weeks, and my surgery is due on the 8th. I did cheat a bit, but only with "smart" stuff, mainly from my reading on the subject. Non-root vegetables, stuff with very few calories, very little fat and carbs, etc.
I couldn't stand it after a while. The protein shake diet is absolutely horrible on my body and it's incredibly difficult to go through only having that. The problem isn't so much cravings as it is total loss of energy.
I was still smart about it, though. I once replaced a shake with a grilled chicken breast (no skin or anything, just the white meat), and I sometimes (like once a day) eat a branch of celery, etc. Nothing big, and from what I've read it falls in line with what most surgeons recommend before surgery anyway if you're not doing the protein shake one. I also figured out that three weeks is a HELL of a long time, and in 2 weeks I've lost about 10 pounds just by doing it. I'm pretty sure my liver is going to be fine.