Well, I used Propane in my Metal KJW M9 today with fantastic results.
It cycled crisply and had good range. Same as Green Gas performance wise. I paid 2 bucks and some change for the propane tank, which holds as much as a standard green gas can.
The nozzle is like 30 dollars, and pays for itself after 2 tanks worth. Even at discounted prices, a can of green gas can cost as much as 10 dollars. At 2 bucks per can, and with good results, I'm using propane from now on! (Well, except when I need that little bit of extra power from Red Gas)