Zoner is not a competitor for XnView (not an image viewer per se), but instead is more of a Photoshop/Paintshop Pro style of photo editor. Some previous versions of Zoner were not "up to snuff" in my opinion, but this version is really excellent in design, execution, and functionality. Here I primarily address the Zoner browser functionality.
This is the only photo manager/editor I know of that supports two (2!) side-by-side browsers. Drag and drop between the two thumb panels is supported.
Zoner has a tabbed interface and offers strong customization of things like labels, text overlays, etc. Also, each thumbpanel (in the browser tab) can show both thumbs AND column headers (a la TC) so that sorting thumbs by various metadata fields becomes very straightforward (just click on the column header, even if thumbs are showing). Excellent.
It's as if the Zoner guys looked at the XnView Layout 7 browser and threw a million programmers at it, addressing many of the requests which have been posted in this forum.
The editor functionality is also strong. Open an image in an editor tab and check out the "Advanced Resize" dialog, for instance.
- As in the past, the Zoner browser supports dual thumb panels, but not dual trees or previews.
The various skins (actually, just color schemas) apparently cannot be modified. IMO the dark schemas are too dark and the lights, too light.
No "exclude" folder list... indeed, little control over the thumbs cache except location.
No thumbnail size slider.
Thumb labels are centered only.
No versioning either, so you Lightroom fans should probably give Zoner a pass.