The first impressions on the Opus are discussed (in English) on the meetup thread on the French forum:
https://www.mobileread.com/forums/showthread.php?t=50232
I thought it was very nice, very small and a nice shape. I was afraid the screen would be too small but it's actually only slightly less wide than the cybook's (and somewhat less high), so the length of a reading line is still good. It also supports automatic switching from portrait to landscape mode.
It does not support mobi, but epub (and html and txt and pdf? not sure about that).