I don't know. I know 4 people that have them, and they love the things. I only have about 650 books on mine. Better PDF support is the only thing I really need. I was disappointed when the firmware 'upgrade' restricted my access to the files on the box.
I'd like better MAC/LINUX support, but I usually just load files as .rtf to the SD card, and go from there.
Has anyone had a problem where an RTF file looks good on the host machine (in this case, a Powerbook G4), but refuses to load onto the SD card? Seems to be without rhyme nor reason, but is annoying.