I would like to know what the pros and cons are from using a setup like this?
I use the 45deg viewfiner with the Seatool D300 housing. Compared to the standard viewfinder the pros are it's much easier to use and you get a better view of the image. Cons are the obscuring of the screen. Also at first it's slightly odd to be looking down at a 45deg angle but to be seeing horizontally. I found for the first dive or two i tended to rotate the camera around the line of sight from my eyes when panning. After a couple of dives i got the hang of it and start panning the camera correctly. Now it's automatic and i don't even notice it.
I havn't tried the Inon straight viewfinder so can't compare it, but I imagine the 45deg viewfinder would be better for shots where you need your body to be near horizontal, e.g. lying on the bottom.
Do you like it? For which shooting situations does it not make sense to use it, if any?
I like it and use it for all dives.
Does it cover the back display too much, or can you still easliy see all of the image captured?
This is probably the only negative point for me - the viewfinder is in the way of the screen. I have to rotate the camera 90deg forward so the screen is facing up to allow me to see it.
Can the owner install it or does it need to be done by Seatool service?
The onwer can install it and remove it quite easily. I remove it for transport and then put it on at the destination. It takes maybe 2mins to install or uninstall.
Do you consider this a weakk point on your housing, for a leak point of view?
I don't. It locks in place very firmly.
jon