Just picked up a 300. It doesn't have the zooming/panning feature. You can just increase the font-size, which strips out tables/images. You can also reorient the screen, which maintains the layout on size (S).
Though I haven't tried it, I think panning a PDF around on the 600 would be better, but still be less than ideal. So i saved my money, and am sticking to ePub books (O'Reilly and Pragmatic Programmers).