I can't speak to the PRS 650 model directly, but I do have its immediate predecessor the PRS 600. The touch is absolutely fantastic, the built-in dictionary has been useful more than once, and the ability to highlight sections or take notes has been handy. The 650 is supposed to have an even more responsive touch, so it should be even nicer.
I have seen the 650 screen, and it's noticeably better contrast than my 600, without the shiny coating that the 600 needed for it's resistive touch screen.
Adding files is pretty easy, you can use the sony software, the free Calibre software (which can also convert ebooks between various formats if needed) or the 600 (and presumably 650) will show up as a mass storage device, and you can just drop ebook files into the "Documents" directory on the device itself. Additionally you can use a Sony memory stick, or an SD card for more storage and books.
Hope this helps!