If you are thinking about buying an older or used e-reader such as the Sony Pocket Reader, the Kobo Reader, the Barnes & Noble Nook, or the Amazon Kindle 2 you should see this e-reader review and breakdown.
This E-Reader comparison video shows features important to the individual and displays how each is implemented on the two e-book readers that have screens unimpeded by an extra layer of glass and which happen to be the two biggest selling e-readers over they past year — the Kindle 2 and the Nook.
This reviewer gives great advice on choosing wireless options and lists the individual advantages of each e-reader over the other.
If you are looking for a newer e-reader look here. Kindle Wi-Fi or Kindle 3G.