I use one. I prefer using it if I can in preference over the dark cloth when working with 4x5. With the dark cloth I have to elevate the tripod. With this I can keep the tripod lower which gives me better stability.
I even hold it to the back of the ground glass on my new Intrepid 4x5 even though it doesn't lock in place like it does on my Cambo SC.
I wear glasses and checking critical focus on the small 4x5 ground glass can be a bit trying using a loupe. Brightness works well for me. The interior mirror is cracked on mine and still works just fine. It is one of the accessories that comes along with me regularly, even on hikes.
I use one on my Cambo Wide with 65mm f/5.6 SA and it shows the whole screen clearly and focussing is easy even with such a wide lens, only thing is I have fitted a Fresnel which makes for nice even brightness through the finder. I love how it rotates easily but never when I don't want it to.......
Highly recommended, only wish I could adapt it to my Super Technika III.
I have one adapted to my Sinar F2. Convenient, lightweight, can see the entire glass. It is very convenient most "necessary" when hiding under a darkcloth is not advisable such environments - not just urban.
I have that one, and also the straight version for eye level. Love them both. If the fresnel isn't matched to the lens there may be darkness in the corner, but I haven't found it too limiting.