If you are looking for a good low-cost EInk Reader, then either the Sony PRS-300 (released by the end of the month) or the Hanlin V5 (BeBook Mini) are the front runners. The Bookeen Opus, also with a 5" screen, is significantly more expensive. We don't have formal reviews of either one, but they are both based on earlier 6" models. Both will support mobile Adobe Digital Editions (ePub and PDF), with Sony's support probably more mature (but Jinke improves its firmware with frequent incremental updates, which Sony does not do). The Hanlin V5 has more hardware capability (e.g. SD card, faster processor and newer display processor) but will probably be a bit more expensive.
See Wiki: E-book Reader Matrix
for a few more details.