+1000 !
In Arnold and Redshift for C4D, it is a standard feature. It makes precise texturing very easy.
I was disappointed to find this is lacking in Corona. Even the new R20 node material editor has it now !
I use the same trick as Nkilar, but as the viewport preview is often not accurate, I use instead the emission channel to preview. Not very practical...
In R20's node editor, the little "s" button previews the node on the material
In RS you use alt+w to connect any node to "output"