All in all they seem to be a fair chunk cheaper than elsewhere.
The reason I was looking is that I'm reading book one of a series (on paperback) again and I'd like to read the whole series. I have 9 dollars of micropay to start off with, but the whole series will come out at about $100 or so. I'd like to pay as little as possible for it.
So I'm thinking that I should make the cash purchases just now while cash gives a 45% micropay rebate. But I should hold off on making micropay purchases, since they currently give 0%. By the time I'm ready for further books in the series I'll be able to use micropay, and frankly it can't be any more expensive than a 0% discount, perhaps by then they'll have changed things and I'll get some kind of rebate on those purchases.