I tried the PRS-600 and did not buy it for two reasons: the small screen size and the time it took when viewing zoomed pdf (large size pdf).
If Sony gives a new generation e-reader with a faster processor and a better contrast (keeping all the great stuff about the current generation devices, i.e. interface, touch-screen, sd card slot, etc), then it will be a great alternative to the Kindle DX.
As for now the promised new SiPix devices (most notably the Asus DR-900) seem to be evaporating...