I just got back from a week at an all-inclusive. And I didn't gain a pound. I ate a really good breakfast, omelets with all the fixings, buffet lunches, lots of jerk chicken and pork, and nice dinners. Plus I drank like the proverbial fish.
I stuck to rum and diet coke, scotch, white wine, and the occasional martini.
I found it very easy to stick to the plan. There were lots of options, and I just avoided rice, potatoes, and bread.