This summer I bought the 45 degree viewfinder from Inon for my Hugyfot underwater camera housing. I took it to Crete for diving mainly for better macro photography. It took me some dives to get used to it because you have to get comfortable with looking at 45 degree and sometimes you miss the subject. But after a couple of dives it proved to work very well. My diving position for macro shots was greatly improved and with a good buoyancy control I could stay horizontal in the water and still take pictures of subjects. Two fingers on the rocks was enough to stay still and take the shot with my other hand. I did not have to sit on the bottom or rocks anymore to take a macro shot, which I hate because as a diver you should not leave any mark on your environment.
The viewfinder is not meant to take it off between dives. I took it off only for traveling because it adds about 7 cm extra thickness to your housing which id difficult to pack. Inon supplies a special tool for taking the viewfinder off only for shops but my dealer supplied me with one.