While I rarely buy the first generation of anything, this sounds interesting once they get some of the kinks worked out (both in the tech arena and the pricing structures), and once the price comes down a bit. I think this will have broader market appeal than the Sony Reader because of Amazon's built-in clientele and distribution association.
Neil Gaiman comments in his blog, here, about the device. He says that it's easy to load your own content.
I'm not ready to buy (I'd definitely like to see a color screen, for example), but I'll be curious to see how it evolves.