I have, and it's rather good - there's a filter for free books, too. You can download to Kindles, Nooks, pretty much anything by the looks of it!!
So how many books can a Kindle take? Mine seems to be groaning under the weight of books... And since I discovered the 'Collections' facility, I don't archive them anymore. Ho hum - at least my book shelves are no longer the ones groaning, I suppose!