A couple of points to add (wrt the Sony 300--have not seen an Opus in these parts):
The 300 displays ADE books borrowed from my library as fully-justified text, at all 3 font sizes.
I have over 100 novels loaded on my 300 (probably close to a year's worth of reading) and still have 360 MB free, out of 443 MB. I will never have a need for external memory cards.
If you are like me, and just want to be able to carry a large variety of books with you to read, then the folder option may not be important. I have no problem quickly finding the book title or author I want with just a couple of button presses. OTOH, if you want to carry an entire huge library around, including tons of books that you won't get around to reading for years (if ever), then you may want to consider a device with external memory and folder options.
Oh yeah. And apparently the Opus crashes periodically, from what I've read here. I would never want my dedicated ebook device to crash or require constant recharging. I want it to act more like a book!
