I've seen Roberts mention once or twice on his blog that he couldn't get a controlled experiment approved by the review board. (No details.) Also he seemed open to the diet just not working for some people.

Some variations haven't been mentioned here, like adding random combos of spices to your food to mix up the flavor. What I settled on, what worked for me without any unpleasantness drawing on willpower over time, was to pour flaxseed oil on toast and eat it noseclipped.