Bought it.
Now I have to figure out the easiest -- preferably automatic -- method to put my books in Dropbox so that I can take advantage of device syncing.
(I've all but abandoned managing my books offline. Now that all my readers have wi-fi, I download my books directly from the sites where I've purchased them. Once or twice a year I'll go out and round them up to save and organize in Calibre, but my library is all on my external hard drive, not in any accessible cloud. It looks like I'll be changing that -- this will make it worth the effort.)