As diabloNL says the the housing is of cheep looking plastic, the PRS-505 is undoubtedly a much more robust and better quality product. It is also a lot cheaper, it cost me £150 where as the Gen3 cost £245.
The PRS-505 when connected to my computer shows as an external drive so I do not need to use the Sony software to load books. If that does not work you only have to remove the SD card and connect it directly to your Mac to add books. You will still have the problem you would not be able to load Sony DRAM books.
Personaly I did not find the margins a problem on the PRS-505 or the PRS-500, this depended on the formatting of the book you are reading.
It's a shame that there is no way to to link to external document sources such as other documents on the reader. Looks like I will have to wait for the upcoming firmware update whenever that is.
For me the one major advantage the Gen3 has over the Sony Ready is it's MobiPocket support and probably the only reason for choosing this as my reader of choice. If the Sony reader supported MobiPocket then I would not have considered the Gen3.