Perhaps this is sufficient reason to connect my KOreader to the internet for the first time...
If my PocketBook had OPDS out of the box it would be the perfect device. What has your experience been? Why are you using KOReader?
My experience has been "fine" - I don't use anything fancy like dictionary lookup or OCR, I just put epubs/pdfs on it and browse. I find the menu UI pretty good, it's a little unintuitive but there is a short guide that covers everything I need. There is no easy way to browse your library (there is a file browser but I don't like that). Ditto I don't find browsing through a book particularly intuitive, but it has page seeking and you can scroll through pages.
For casual reading, I think it's just fine.