I just discovered ebookprice.info towards the end of last week. I had been writing my own thing last week (
http://book-find.appspot.com) to aggregate the results from a bunch of sites, but I've only gotten 4-5 stores included so far. I had gotten tired of going around doing the same search at a bunch of different sites, and I wanted to try out the Google App Engine.
However, it seems ebookprice.info already did the equivalent and did it better. I'm not thrilled with their interface, but they include more stores and don't have to worry about individual stores timing out.