Hi,
yes, its a nice tool
It was actualy working for me also with mental ray,, in a little strange way, but it was
when i hit "render" in mental ray and after close,, the ActiveShade is working but when i turn it off and try again its not working with mental ray
strange
V-Ray have RT, i dont know from which version, but it generates the "final" result directly to your viewport,, Nitrous driver works similar but not so good like RT in V-Ray.
What you can do is only to set-up your render to very-low settings,, to tweak your lignitng and so,, and just after do the final render,, like this you could do a render in a minute,, i know still alot of time
