As great as these things look, they're quite dissapointing when you try to use them. The black GBTech comes with the wrong kind of fill valve (single port):
http://www.airsoftcanada.com/showthread.php?t=27456
Because of the single port valve, you can only inject about 8g of propane into the GBT charger which is about what one WA doublestack mag can hold in a fill.
I'm not sure about the VFC model. They may have dealt with the fill valve issue.