For the purchase cost - ~Â£140, you can purchase a good CO2 kit.
For the replacement block (which doesn't last more than a few months I've read) cost - ~Â£25, you can buy a few KGs of CO2 gas.
Poor availability, spare parts, reliability etc are all problems. And there is no way it can produce more stable CO2 production than a regulator.
If they were any good they would be in every aquatics shop and on every aquatics website. They're not. 'Nuff said!
Yep, I agree with gratts. The fact that you don't hear much about it is an indicator of it's popularity. Perhaps the least appealing attribute of this system is the fact that you'd have an object the size of a brick in your tank. For very large tanks one might be able to hide it but otherwise it's like having a gorilla sitting in your lounge.
I suppose it's an option for those who are paranoid about having a gas cylinder around....