As I understand it, the current is pitiful for the size, but the longevity and total power output is insane. Like, you start with a cell the size of a soda can, it only puts out 250mW or so, but 500 years from now it will still be putting out 125mW, whether you've been using it or not, and generated close to 1MWh (Mega Watt hour) worth of electricity in the meantime.
It's not really a battery as much as it's a nuclear fuel cell extracting power directly from radioactive decay. Efficiency is crap (fraction of 1%), but there's so much effective energy in radioactive decay you can still do something useful with it.
--Dave