I hear it works but it takes a lot of effort to get into and stay in ketosis. Even a couple grams of carbs can but you out of the optimal zone and leave you feeling tired and unable to concentrate or properly furl yourself and your workouts. Carbs are our preferred source of fuel. Trick is to focus on good carbs and carb timing if you are active rather than eliminate them.
Important points which are a bit nuanced. For many (maybe
@hung9thick) this is intuitive, but I'd bet he still works very hard to keep in such great shape.
Ketosis per se is irrelevant, it's only a marker for relative calorie (and probably specifically) carbohydrate deficit. It's neither a means nor an end, only a marker. The misconception here started with the Atkins diet - he thought fat was magic, carbohydrates evil and ketosis was the means to lose weight. He was close but not correct.
The real enemy is simple carbohydrate intake, primarily in the form of processed sugars and grains. That is, most white food (sugar, white flour, etc) are best avoided.
Fat is largely irrelevant as well but of course, you cannot drink lard all day and expect to lose weight because excess calories cause weight gain, period. The trick is that more protein is definitely better; increases basal metabolic rate, decreases hunger and snacking and is good fuel.
For most people trying to lose weight, there's an easy way to demonstrate this. Attempt to change nothing about your diet except to add two hard boiled (or scrambled or whatever) eggs when you first get out of bed, again at about 10:00 AM, 3:00 PM and anytime you're hungry otherwise. This isn't sustainable generally because it's boring, but I've yet to have a patient who does not see the appetite suppression, increased energy and weight loss within a few days. They can then take that example and start to modify the other food they eat during the day to lower carbohydrate intake.
A second easy demonstration is to lower total daily carbohydrate consumption below 20 grams or so. That's very difficult to do and probably not sustainable, but again, I've never had a patient who did not quickly see the effects.
For readers, the South Beach Diet book is a good start, it explains the science pretty well.