I have a Robert White viewing bellows with magnifier for my camera. At first I thought it was a bit difficult to use compared to using a darkcloth and loupe but now I am not sure.
I have been using it for about a month now and it seems to be doing everything that I could with a darkcloth and loupe. About the only area where I sometimes remove it is when I want to check deep in the corners with a loupe. (not that often for me).
The three key advantages I see for this is.
1. No need to remove, quick set up and adds protection to the groundglass
2. My head is always in plain sight so I do not feel like somebody could sneak up and grab something or perhaps accidently fall over me.
3. The lousy wind is not an issue any longer when focusing. Still not desired when shooting but at least the dark cloth is not closing in front of my eyes as I try to focus and adjust.
4. I do not believe that I will now be fogging the groundglass during winter shooting. (unproven but a reasonable guess)
As of late I am starting to get really comfortable with the viewing bellows and doubt very much that I will be using a darkcloth much into the future.