For corny kegs the fast carb method works well in my experience if you're in a rush. I have tried one of these lids...works but not as fast as the fast carb method, which, with a corny, you can fully carbonate in about 10 minutes, with a bit of effort.
So worth the investment, well not sure...if you're in a rush then the fast carb method is faster, if you're not in a rush it only takes a few days to carbonate normally if you bump up the pressure for the first 36 hrs before dropping to the correct pressure as mentioned above.
In anycase I think I've been using carb stones incorrectly, not that I use them alot. I assumed you applied the pressure to the closed system and as the beer absorbs the co2, the co2 input through the carbstone is done so in the form of tiny bubbles - or ramp upto your set carb pressure in a few steps. I think this is correct in principle, but not sure how much quicker it is compared to normal headspace.
However recently saw from a commercial brewery they do it differently. They use a spunding valve set to the correct carbonation pressure, calculate the weight of CO2 needed to carbonate the beer, place the co2 cylinder on scales, then apply sufficient regulator pressure to get the spunding valve going, then spund through the carb stone until the weight of the CO2 cylinder indicates sufficient co2 has gone into the beer, then you're done. I guess this method is the quickest if your vessel is too large to use the fast carb method.