I've never seen the Baader Mark V but reputedly it is as good as anything else available (Google Roland Christen's comments about it and read some of
Tamiji Homma's posts about them on CloudyNights). I do have hands on experience with the Denk II and Denkmeier PowerSwitch system. The short version is that I like them very much! I generally do
not see the 3D effect that some people describe when using binoviewers (I do when looking at the Moon but not when observing any other object). I find observing with two eyes considerably more comfortable than mono viewing and this is important because the longer I look at something, the more detail I tend to see. The Denkmeier PowerSwitch works well and is a convenient way to obtain multiple different magnifications with the same pair of eyepieces.
Now for two quick general binoviewer suggestions:
(1) Try to look through a binoviewer before you buy one (even a cheap one will do). Some people just cannot get the hang of merging the two images and you'd hate to spend lots of money and find that you are one of them!
(2) I recommend using mid to long focal length eyepieces (>=10mm) with a binoviewer. If you want higher powers then use some kind of Barlow type lens system (whether it be a Barlow, an OCS lens from the binoviewer kit or the Denkmeier PowerSwitch system) to increase the magnification further. Doing this makes life easier for the binoviewer (because less amplification is happening within the device itself) and makes merging the two images easier. I use three pairs of eyepieces with focal lengths between 12mm and 21mm with my Denk II.
So far I've used my binoviewer with a couple of different refractors and I love it for the Moon, planets and brighter DSOs.
