Personally, we don't use the credit cards because we wouldn't use them enough to make it worthwhile. And the whole let the plastic in my wallet lead me not into temptation thing...
But along the same lines, that's kind of what we do with our PayPal cards. Our checking account earns interest as long as we use the bank's debit cards 12 times in a month. That's easy to do: a coke here, can of Altoids there, etc. PayPal gives 1.5% cash back, and it draws from your bank account if you don't maintain a balance. We put everything we can on the PayPal card and usually get more than a tank of gas for the Mini in the cash-back amount. We've filled up the truck when the big annual stuff like taxes were due.
We use amex as our cash replacement, you have no choice your bill is due in full at the end of the month so we literally monitor it's balance and our bank account just as if it were a debit card. When we go to Germany in a few years one of our tickets should be paid for by point's balance and the other one should be signifigantly discounted.
In the few instances Amex is not accepted we use the discover card. Put he taxes on it this year.