I don't like the rebate system either. Books on Board uses it but not as much as FictionWise. Very often the price at Fictionwise after rebate is the same as the base price at Books on Board, which makes BoB cheaper really (I don't get to pay forward on my next purchase, I just pay the price of the book). Unfortunately that's not always the case, sometimes FW is cheaper. And of course the 100% rebates at FW are something to look out for, you get 2 books for the price of one. I'm still rather cautious though, because this feeling of getting a deal can sometimes lead me to buy books I wouldn't have bought otherwise. Which of course is the whole point of the system
Also, some books are available at one store and not the other, or in different formats. So I end up visiting both sites all the time, but I buy at BoB more often because it's cheaper most of the time. I hate the site interface though, FW is much easier to use.