I just got the Inon 45-degree viewfinder for my S&S D300 housing, and just made 12 dives with it last week in Lembeh. On my housing, it definitely obstructs the view of the LCD on the camera back. If you rotate the viewfinder for a Portrait shot, its better, but still partially blocked and topside, i found it to be quite annoying.
Underwater it's better. I didn't really notice the blockage much. It's still pretty easy to see the LCD if you just tilt the camera down a bit...
I got to admit the viewfinder takes a lot of getting used to, at least for me... With the 105mm lens, I spent a LOT of time just trying to find the darn subject... the 45 degree offset was a real challenge for me. I did find myself getting better over time, and once you do find the subject, the view is great!!
I was fortunate to be diving with Takako Uno and Stephen Wong, and they told me there is a 20-dive learning curve with the viewfinder, so I am not too alarmed... 
Good luck, and if you get the viewfinder, please share your experience!