Addall just searches book sellers. The cheapest/most ready available most often is Amazon new & used. I've gotten about 50% less than the bookstore price for the majority of books I've needed.
I'm researching my own books right now actually. Fall semester I need 4 books. Two CS and two EE. One is brand-new and one I can probably borrow anyway, so I'll only be saving about $65 this time (still nice). I pretty much only use Addall, because it searches them all anyway. Pretty much all of the sites listed here are searched by it. But again Amazon usually comes up the cheapest IME.
edit: Thus addall isn't slow, it's up to the individual seller. Some of mine have taken over a week, others in 5 days. Don't wait till the last minute to buy them, that's all. Well if you do, just go to the bookstore and charge the needed books to a 'card, then when you get your shipment, return it to the bookstore. I think pretty much every bookstore has to accept refunds for the first week or two while people drop/add classes.