Ketones are metabolic fuel, not a supplement per-se. You could say it’s a macronutrient like carbs, protein or fat, it’s just that we don’t tend to find them in the wild. On a low-carb diet, your liver makes ketones for free. Ketone-salts are much cheaper than esters. MCT oil is also metabolized directly into ketones by the liver, I believe on any diet. The unique promise of ketone esters is they can raise your blood ketone levels higher and faster than most other methods. An ester can of course be stacked on top of any of the preceding methods. On a calorie basis, there’s not a lot of energy, but at high outputs a little bit extra can make a big difference. They’re very easily metabolized and require less oxygen per atp produced, which is crazy-awesome when you’re on your limit already.
I already do all of the above except the ester for basically all races. I might consider the ester, if they were cheaper, in place of the ketone-salt. But because of the cost I’ve not tried it, and probably won’t. I certainly wouldn’t train regularly with it or anything like that.