You could just install it and see what you like best, it is not exclusive of the stock reader.
IMHO Koreader is, with respect to the stock:
Pro:
- much more customizable in terms of screen display and text rendering, including the possibility of working around bad book css definitions
- remember settings per book, instead of globally
- includes a lot of goodies such as inverted screen mode, customizable progress bar, OPDS downloader, and much more
- handier for dictionary lookup (but doesn't support the .dic files which the stock uses)
- handles odt files (besides of course epub, pdf, ...)
- handles correctly bidirectional mixed text
- well maintained and frequently upgraded
Contra:
- can't read DRMd files [no bought books, no overdrive etc. unless you de-DRM them]
- lacks some OS integrations: reading progress unknown to the main librarian, screen blanked on sleep
- somewhat slower than the stock
There is also coolreader, which is somewhere in-between: customizable but not as much as koreader, supports .dic, does NOT support LTR, no longer maintained for pb.