I find it more or less useful than the standard groundglass. Its main advantage is the ability to rotate it 90 deg. Basically its
going to do the same function as the standard groundglass cage. Its nice for studio work where your lighting is fixed and you want to move around for composition. It suffers badly from pincushion distortion, but you can get used to that. Of course, no light metering functions. They're not very common - if you find one used in good condition at a good price it may be worth grabbing. If you don't do verticals, there may not be much point, use the reg waist level finder.