I've posted this many times allready, but for me the 600 so far is the best pdf reader (if you consider price, portability, availability in stores...).
For pdfs with heavy formatting (text + pictures and/or graphs) I use the zoom where you can define the zoom level with the slider and then pan around. Some see it as a drawback that the zoom level resets with every page turn. Well, sometimes it is. Sometimes it's useful. Take a magazine with multiple columns and pictures and such. Not all pages have the pictures on the top or bottom of the page. Sometimes the text columns are on the right, sometimes on the left. So the layout alternates from page to page. In this case, resetting the zoom level is great as you first get an overview over the page and then zoom in on whatever you want to read. Sure, it would have been awesome if one could set how zoom behaves per book.
For regular text only books, the S, M, L reflow-zoom works well enough. There's the occasional linebreak in the wrong place, but mostly it works pretty good.
Maybe this helps.
It always depends on what you want to do with the device. If available, I would go for the 600 in letter size (well, this isn't available yet).
