There are definitely some nice changes in 2.3.1, my favorite being the redesigned home screen. Unfortunately basic readability problems sent me back to 2.1.5 (plus plugin) within a day.

UNACCEPTABLE SHOWSTOPPERS
- Reader no longer reliably remembers where I left off
- Favorite fonts render italic as bold (possibly a font naming issue)

ANNOYANCES
- On first opening some epubs, an unselected font flashes for a second before settling on my choice
- Loss of "back" button (link to library screen from reading menu)
- SD-card epub covers draw slowly (no longer cached?)
- Progress bar added for kepub but not epub
If you haven't yet upgraded, consider a full backup first so you can revert if needed. Good luck with 2.3.1!