Excellent.
I must have 25 of the 20 pounders (I think that's what you mean too) bought at garage sales.
It is amazing how much prepper 'stuff' can found at garage sales on the cheap. I'm probably up to 30 cases of canning jars to @ about $1.25 per case average. Even if I never use or have to use them, they'll still be good barter items.
I've read an estimate that all the cooking for a month for a family of four can be accomplished on one 20 pound propane cylinder.(that's pretty good, if true)
Some time ago the EPA lowered the capacity of the 20# cylinder to a max of 15# due to expansion of the gas if exposed to heat. They still call them 20 pounders but, all propane tanks, regardless of size, are subject to rules based on safety and can only be filled to about 80%.
Also the tanks need to be recertified so if your tanks are older than 2000 you might wish to do an exchange. Here is a good site for propane info.
http://www.propane101.com/propanecylinderfilling.htm