How is Bitcoin supposed to serve as currency when it is all but impossible to spend at the moment (12/2017)?
You were probably trying to pay on the blockchain itself. That's like trying to pay for lunch by moving dollars through the Federal Reserve. You should use some system designed to move small quantities of bitcoins cheaply such as Coinbase.

We don't yet have good decentralized payment systems for bitcoin, so currently bitcoin is primarily being used as a store of value. Think of bitcoin as more like gold than dollars. The Lightning Network is one proposal to provide a way to move small amounts of bitcoin at high speed with low cost.