I'm using a Maxwell on my Chamonix and like it quite a bit, but it cost almost $400 IIRC. The big advantage is being able to see in the dark corners with wide angle lenses, and the differences in those areas is dramatic. I have a Beattie that came on a 2D 8x10 that's lovely, and a couple of other ones on other cameras, one being a cheapie Edmund Scientific. The cheaper lenses are coarse and thinner and the images don't look as sharp in the magnifier, but they do let you see in the corners in beneficial ways.

My Maxwell was a difficult expenditure, but my rationale was that this was going to be the most frequently used part of the system, and would likely make contributions to my compositions or at least help me to avoid painful mistakes. Its kind of like cleaning the windshield...