There is an extensive discussion here, which we don't need to duplicate:
http://www.flyertalk.com/forum/manuf...amex-blue.html
However, the rewards program is straightforward:
Up to 5% Cash Back at U.S. supermarkets, U.S. gas stations & select U.S. drugstores (Everyday Purchases). Up to 1% Cash Back on other purchases. For your first $6,500 in purchases in a reward year, you will earn 1% on Everyday Purchases (5% thereafter), and 0.5% on other purchases (then 1%).
This means that each 12 months you will spend $6,500 on which you will earn only 1.0% (or 0.5%), and you would earn more if you spent those dollars with another card.