I personally find the preview nearly useless. When small (to keep workflow speed up), you can't see enough...just easier to do a preview render in the viewport. When large...well it can't be detached so you lose viewport space, so yet again useless for my workflow.
When E-on allow more user GUI configuration (i.e. moving and detaching of panels), then I may find the preview more useful.
Another one of it's downfulls is that it looks like it uses the same rendering engine as the main renderer, which isn't great for speed. It would be nice if you could control what channels you can render within the preview so you could increase the speed at a higher resolution...such as Modo's Iview.

Cheers

 

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