I have a mac and actually have the 505.
It works great, really. libprs500 software, for managing books and such from the internet and converting documents to sony's format isn't that bad. Not as polished as a mac user might like/be used to, but it is a free product.
As for Sony Connect...you do need windows for this, sadly. I use VMWare fusion or Boot Camp in this case. If you have a PowerPC Mac tho..may want to get a kindle. its not pretty and the screen isn't quite as nice as the Sony devices, but you'll not have problems buying books.
I don't know anything about the Gen3.