I know the PRS-600 is supposed to be faster for page turns, but it didn't "seem" lots faster when I "test drove" it at the store.
However, I find that I - as a 1.5 year PRS-505 owner - am constantly pressing the "page turn" button too soon. I'd obviously trained myself to turn the page approximately 3 lines before I finished the page, which would let me read uninterrupted with the slight delay in the page turn speed.
I keep having to go back a page to read the last line in my 600!