There's a gas station/convenience store/wine shop combo place near here that refills 20 lb cylinders only. They have a scale they set your tank on, and when it reaches a certain weight, they stop. I don't know what weight they have it set to. And you pay a set price for a refill, so even if you have some propane left in the cylinder, you still pay for a full cylinder.
It works out OK for the cylinders we use on our grill, because I never remove them from the grill until they are empty. But you lose if you have a partial cylinder.