Thats good to hear - most of the flywheel densities that I haven't been so high. I can't find any commercial wheels of that density for individual use, tho (many references to start-ups making them for central power utilities).
Satellites are a special case, as they have enormous budgets, plentiful solar power (so cycle lifetime is more important than capacity), and have the additional benefit of using the flywheels as a rotational propulsion device (usually in counter-spinning pairs).
It seems that commercial flywheels are about 7 times more expensive than batteries per kWh. The article mentions 24 batteries at ~$1,500 each providing "3 days" of power at an average of 2500 watts, so 180 kWh for $36,000. A recent blog post regarding Beacon Power flywheel systems quotes them at ~$1,400 per kWh, or ~$250,000 for 180 kWh.
Satellites are a special case, as they have enormous budgets, plentiful solar power (so cycle lifetime is more important than capacity), and have the additional benefit of using the flywheels as a rotational propulsion device (usually in counter-spinning pairs).