I fall into this group as well, and my sentiments almost match those of Arapito's exactly.
My biggest concern
'had been' page-turn speed, but I think the 505 turns pages faster than I use to with conventional books. When I read while riding a train, standing, I'm turning the pages with the same hand that is holding the book, fighting to turn the page with just my thumb. MUCH slower than with the simple button push of the 505.
The design is elegant (being silver,) functional, and simple. The memory certainly much more than I really need. (Right now I have 118 books loaded.) The device seems robust enough for the hazards of daily carry inside my shoulder bag, jacket pocket, or tucked into my camera bag. The screen is certainly clear enough with good contrast to make it easy on the eyes. I'm happy with the cover that came with it, but will make another out of better leather just because I enjoy leather working.
If there was one single thing that I'd change/add etc., it would be easier navigation between books being read at the same time. Perhaps I simply need to make them into a single collection, adding or deleting as needed.
Overall, I don't see myself switching brands in the near future. With the ease of Calibre and the overwhelming numbers of books out there, this is a wonderful addition to my lifestyle.
Stitchawl