To give you an idea of how different it is from commercial products: it actually tracks reading in a useful way. It shows a chart of how long you have spent on each page, so you can figure out which parts of a book you have not yet read. That is really useful when jumping around technical books. If you are interested in tracking your general reading habits, there are handy views that shows which books you have read and when you read them (either by time of day or across a month).
As for reading PDFs, well, eInk has its limitations and KOReader does it's best to work around them. If you want to read a multicolumn paper on a small screen, you can configure it to go down one column then right back to the top of the next column. If you want there to be overlap between the screens when panning, you can configure that. You can also have it display which parts were overlapping, so you don't get lost when it displays the next part.
There is tonnes of other stuff in there. I just mentioned those two because I use them the most. Overall I would say it feels like KOReader was designed by people who want an amazing reading experience, rather than by people trying to sell novels.