From my personal experience (18+ months with a Sony 505) and what others have said the Sony readers still seem to have the slight edge in pure readability/use vs cost (but the knidle pips it for ease of buying/transferring bought content).
I love my PRS-505 enough so that if it died tomorrow i'd be trying to get a 600 ASAP, as I've got hundreds of books for it (I'm fairly lucky in that my tastes in books include a lot of the stuff that Baen offer at
www.webscription.net*, and classics that are out of copyright), and it's no harder to read for me than a normal book (easier than some of my favourite hardbacks**).
My one complaint with the 505 is that when you've got a few hundred books loaded onto a memory card it can take a couple of minutes to get itself organised when you disconnect it from the computer after adding more/charging it (the latter problem can be easily solved with a dedicated charger).
*And often as promo CD's with their hardbacks (I'm hoping the new Honor Harrington book due out in a couple of weeks will have one of Baen's CD's, as they often include 30-40+ books, including previous ones in the series)
**A 600-1000 page hardback tends to weight a bit, and be a pain to read comfortably whilst protecting the spine, I have to hold back an urge to kill people who put books down open when they stop reading, or even worse fold them back to read (that deserves them to be hung drawn and quartered, especially if it's one of my books, or a library book).