i find the fictionwise system to be very handy if you buy or pre-order a number of 100% rebate ebooks. last weekend i bought a couple of more expensive ones that i wouldn't have bought ordinarily, then was able to turn around and buy 5 regularly priced ebooks with the rebates a minute later. i basically got the 2 more expensive ebooks free. free is good
i actually saved a couple hundred dollars in the last year and a bit picking up ebooks during large or 100% rebate sales before b&n bought them.
but i find the prices at books on board are good without a membership, and their system is pretty easy to understand.
personally i like incentives like this - it's like frequent reader miles