I'm using Freda as an ePub reader on my Win 8 tablet (Lenovo Twist) - and so far I am not really thrilled with it. Seems to work OK.. but... The long (16:9) screen doesn't really work in portrait mode - nor am I enthralled with the display of pages or fonts.
Kobo is supposed to read ePub3 - but lots of luck getting it to read sideloaded books.. I haven't figured out how yet. Microsoft aren't too keen on you digging into the Windows apps folder and have locked it down pretty hard.