I have found that National can be significantly more expensive than Budget, Dollar, or Thrifty on shorter, workweek rentals. That said, my company requires us to take out damage waiver -- and once that's factored in, the National
Business Rental Program frequently beats everyone else out there.
I still find National rates to be very good for weekend leisure rentals, though sometimes I need to apply a coupon code.
Also, be sure to always check prices with the Emerald Club contract -- occasionally that is significantly lower than the rates reported through Kayak or its ilk.