Jetor,
I have the Nauticam 45 viewfinder. It's been very reliable, I truly enjoy using it, in fact, can't even imagine going without it and can say it's worth every penny. I've had it for 7 years now, and it's got well over 800 dives and it's still working with no problems. It has an adjustable knob on the side to adjust focusing onto the camera's view finder. Once it's set, you just leave it. I focus it to read the numbers ant that's it.
The only times I have difficulty with it is when shooting macro subjects during night dives. The fact that it's angled, and that shooting a particular situation may call for upside down buoyancy or some awkward position can make it a bit more challenging to frame the subject, since there are just less points of reference to actually spot the critter. Nevertheless, I've always found a way to overcome that situation and it works out in the end. Of course, a 180 view finder solves that, but then the 45 deg. viewfinder lets you get closer to small critters that are at ground level, whereas the 180 won't. It's all a compromise, as you know.... for wide angle, I've never had a problem.
Again, it's only a problem in my case, at night. To overcome this, I shine a red focusing light and try to spot a silhouette, then look for it through the viewfinder and that works for me. My viewfinder has endured lots of use. I take care of it, but then, it's got lots of miles on it and I am very happy with it.
I wish that Nauticam had a better way to fit the thing into the housing. It actually stays in place, thanks to a rubber o-ring. It's crazy. In any case, it's never failed me. But then, I still witpsh it had a more robust fastening mechanism.
I have not tried any other view finder. Hope this helps.
Saludos,
Ricardo