I have only ever used the Ikelite view finder. I like it and have no difficulty using it. That said, the following are my personal notes re coming to terms with the viewfinder.
Adjust the Viewfinder Diopter Adjustment Lever on the camera before putting it in the housing since it can not be adjusted when it is in the housing. Wear the same optics when making the adjustment that you will wear underwater.
Note 1: I was concerned about my ability to compose pictures with the D70 viewfinder as compared to the LCD on my C-5050. Turns out I like the viewfinder better than the LCD. I can easily see the whole frame and looking through the eyepiece really focuses my mind for the shot ... it's like looking through a scope on a rifle. So with no shutter lag it's aim-shoot, aim-shoot. And my compositions are spot on with marine life action shots I never could get before.
Interestingly, looking through the viewfinder with a dome port make objects appear further away underwater i.e. where they really are. This can cause some surprises when, for example, taking a picture of a diver swimming toward you. Looking through the viewfinder it appears that you have time to take one more shot before the diver swims into you. So you take the shot and hen look up to see the diver 25% closer and bigger than they did in the viewfinder.
Note 2: I wear glasses for reading and have gage reader lenses in my mask with magnification for seeing my watch, gages, etc. But I have no magnification in the top part of my lenses and it's this part of the lens that I use for looking thru the viewfinder. The reason this works is that if you adjust the focus of the D70 viewfinder on land without glasses, the same amount of magnification will work underwater without special optics. What doesn't work is trying to look through the lower part of my gage readers. Not only is the magnification wrong but the angle of my face to the camera is also wrong.
Note 3: To adjust the viewfinder, you need to have a lens on the camera and the lens has to be focused. So, put the lens on the camera, turn the camera on, look thru the viewfinder with the same optics when making the adjustment that you will wear underwater, aim the viewfinder at something you can easily see detail in, press the shutter halfway to focus the lens, and then set the viewfinder focus with the Viewfinder Diopter Adjustment Lever.
Bottom line: Adjust the Viewfinder Diopter Adjustment Lever on the camera before putting it in the housing since it can not be adjusted when it is in the housing.