Net Promoter Score
I urge you to read the cover story in the current Fortune Small Business: "Get Customers to Sell For You"
The article builds on the masterful work of Fred Reichheld, and features his invention, the "Net Promoter Score." There are numerous ways to measure it, but somehow one has to end up with a single number: NPS. In (very) short, the NPS is the percentage of customers who are wild about you (pretty damn happy or better) and would recommend you to others minus the percentage who are neutral or worse about you—and, hence, would not go out of their way to sing your praises to their pals.
There are a million twists—e.g., Brain Training—LearningRx, with 66 franchises, has developed an NPS for every employee—with high impact on their performance and pay.