I've finally gotten around to trying out the Studio One Project/Mastering interface. 
Wow!  Full VST support so all plugs are available.
 
What are you folks using?

Ozone 7 Advanced
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
(I'll get me coat)

I don't see any reason for setting up a separate mastering environment, except for assembling a CD, in which case I use SONAR. I have Ozone Advanced, and have played around with using it as a standalone mastering environment. It works well but I found no advantage over mastering in SONAR.

I also use Studio One w/ Ozone & other plugins for home mastering. It burns to CD & has the added DDP feature for delivery to mastering houses.

Nowadays, there's so many ways to build a decent Mastering environment, and I do still hope that Sonar will one day have many features that will make Mastering even better. Right now, the biggest things it lacks are better metering (much of which is easily purchased through 3rd-Party plug-ins) and editing/adding Metadata, etc. Some sessions I do have to use Wavelab, but with Collapsed Mixing techniques some engineers are doing more and more these days, Mastering in Sonar is not the worst idea. Don't forget to ensure your Master fader is always set to zero (0)!

For single track is not an issue where really, but if doing an album Studio One and Ozone Advanced have better tools to accommodate. There are others, but I am only familiar with those two.

Got rid of me Ozone 7 Advanced.
Here's looking at Ozone 8.
And Mesh's coat.