You'll be hungry, you'll be crankier, you'll be flat, small and lack energy.
Save the bit about flatness, these claims are all demonstrably false.
I'm not interested in having OTH explain why it works. Who cares? If you try it, you know it works, and that's all that really matters.
1. Ketosis-inducing diets (there aren't many varieties, but there's more than one) have always worked tremendously well for me as far as stripping away bodyfat, maintaining muscle, keeping energy levels stable, and maintaining a modicum of mental function are concerned. I suspect many doubters are "virgins talking about sex," so to speak, never having tried the diet. And that's understandable, given the U.S. government (via the NIH, mostly) demonizes any such approach to dieting to the extreme, an attitude which trickles down into nutritionists' minds across the country.
I also suspect that those who are complaining of having tried such a diet either didn't do it properly and thus never entered ketosis in the first place -- meandering on the border between 'low enough carbs' and 'just a bit too many carbs' -- or otherwise are referring solely to low carb diets, which are different than ketosis-inducing diets (which are essentially zero carb diets).
2. OTH, what sort of cardio regime do you like to implement in conjunction with your diet?