I timed how long the JBL opened my Don Quixote ePub file (~1.3MB, about 1400 pages, no graphics)---it took about half an hour from an SD card (2GB, class-2). I got the same result when I used an 8GB, class-4 SDHC card. I'm guessing this long loading time is specific to the ePub format (maybe because the entire file has to be read before it can be unzipped?).
The font on the JBL can use a little smoothing, but overall I like my new JBL!