I have upgraded from PRS 505 to 650 and I love the improvements: more contrast, better night reading lamp, no more glare off the chromium bevel, dictionary, note taking, better software, faster chipset, bigger storage. Only small drawback: with the lamplight cover the reader is now too wide to fit in a coat bucket when i go to the park for reading outside. otherwise the 650 is smaller in size than the 505.
Regarding the iPad - it has better contrast than eInk, but it is pretty useless outside in the sun, unless you need a mirror to check your face. Inside it is also a little too big to hold comfortable in bed compared to Sony and it has less battery life [but still more than a notebook]. The iPad shines though when you use it for technical documents / magazines / pdf's where the bigger resolution is capital. Also great for comic books - I use it in landscape mode. I could watch movies on it but i don't like the hassle of using the exasperating itunes interface for managing them and constantly filling up my limited 16 Gb storage.
Regarding the eInk vs paper controversy: I'm fully in the eInk camp, I read through the thread and it appears that most of those who posted against haven't really tried electronic readers and are against on principles. The experience is not really all that different from reading a printed book.
My big selling point on the technology is less tree cutting: I get rather mad when I see the amount of waste when my letterbox is filled with advertising brochures from supermarkets on a daily basis. It's not so much about books as about newspapers and magazines.