How to evaluate a Cryptocurrency (Payment method)

What is the criteria which is is good to use when evaluating a currency (crypto or fiat) and its associated payment systems.

Criteria for currencies

Store of value - how well does a currency hold its value over time,
Unit of account - is the currency highly divisible with great precision, is the currency fungible
Medium of exchange - is the currency widely accepted as payment for goods and services
BTC is a good store of value and a good unit of account. As a medium of exchange it has a long way to go but many improvement have been made in the last 12 months.

When looking at payment system for transferring BTC from one party to another

How many people use the payment system (e.g. the bitcoin client)
The total value of all the payments going through the system in a period of time (hour, day, month, year).
Is the payment system being improved or developed
For example US Dollar is a widely accepted medium of exchange and a good unit of account. As far as a store of value, the US Dollar has been slowly losing its value over the past 100 years. I believe its lost about 95% of its value since 1913. Although this loss is gradual the cumulative effect is large over longer periods of time.