Downside: No grains, beans, dairy; that means no coffee, no chocolate, no soy, no cheese, no bread or noodles or pastries. (Plus sides: bacon, sausages, eggs, figs, dates, walnuts.)
I gradually fell into this diet by carefully noting which foods made me feel bad ~15-30 mins after. I did this over a process of many months starting sometime in February. Finally, just a few days ago, I concluded that I was basically drifting into Paleo of my own accord, and for me to be at my best I had to go from reduced intake of the banned foods to complete removal. I'm still going to cheat every now and then, but I know I'll feel it every time.